Some basic concepts. This is not exhaustive, but may help you understand some of the estoteric terms and concepts involved with eurorack, modular synthesis and synths in general.
All signals in a eurorack modular are technically CV (Control Voltage) signals. All signals are usually between -10/+10 volts, but are more often in the +/- 5 volt range. The main difference between control and audio is usually just the rate of change. Signals that oscillate more than 20hz (20 cycles per second) enter the audio rate range and can heard if you plug them into an amplifier. Therefor in a modular system you can plug audio sources into modules to control them, or plug fast moving control signals to make sound. This makes your signal path incredibly flexible. However, having said that, there are a few standards of signal in eurorack.
The first rule of modular synth is patch anything to anything and see what happens! Generally speaking nothing bad should happen (though this can depend on the design), and maybe something wonderful might happen. Try patching those audio signals into control inputs, see what happens when you patch that LFO or envelope into your mixer. However, the second rule is try not to patch outputs into other outputs, or passively mult (multiply/add) together signals together so that the total voltage is more than 10V. While most designs have current and voltage limiting on inputs, you don't want to risk letting the magic blue smoke out of your module.
As stated previously, be careful patching out of modular into consumer and pro audio equipment, as it may not be rated to handle the (relatively) high voltage range from a modular system. For example a Korg Volca (according to their website) can handle up to 20V on it's sync in, but a Teenage Engineering Pocket Operator can only handle a max of 5v peak to peak. Plus keep in mind that almost no DC and battery operated machines like negative voltage of any kind.
Patching stuff into your modular from external sources is usually safe, however audio will need to be amplified considerably (or everything else mixed very low) in order to hear it, and even gates/clocks may need a bit of gain to trigger modules. You can use input modules and/or envelope followers to make this kind of interface easier depending on what you want to do, though there is usually no harm in trying things out to see what works/happens.
These are your basic pitch based sound sources. They usually have one or more wave shape outputs (most common are square, triangle, saw, sine, but many others are possible, available). Often multiple shaped are available from the same 'core'. Usually there is one or more 1V/oct pitch inputs to control the pitch of the oscillator, and there may be other inputs, such a FM (frequency modulation), PWM (pulse width modulation), Sync (restarts the waveform when signal is high). Generally speaking the range of VCOs is in the audio rate 20hz - 20khz, but can be higher or lower depending on the designer/purpose. Usually VCOs always output a constant sound unless they have a built-in VCA.
These are used to control or shape the volume, or voltage of a signal with another signal. The most basic usage is to control the volume of a VCO by using the signal from an envelope. However you can use a VCA to control the voltage of any signal. For example, you could modulate the height of a LFO using the output of another LFO. VCAs sound quite dull, but can end up being one of the most useful modules you have in your system. Most VCAs have a input for the signal to be reduced/amplified and an input for the amount, and an output with the modified signal. When you modulate a VCA with an audio rate signal, you get an audible effect called AM (amplitude modulation).
These are generally used to shape the frequency spectrum of a sound in some way. The most common filter, the Low Pass (LP) filter, lets through all sound at the top, cuts all frequencies above it's position by an amount based upon the design. It's similar, but different to the effect of a VCA. Most filters have a resonance control which introduces a peak at the point of filtering, and can be increased enough that it creates a fundamental or sinewave at that point, and can be used as a VCO. Many filters have a 1V/Oct input for this reason. Filters usually have 'slopes' to them, usually measured in 'poles', 1 of which is 6db/octave. In a 2 pole Low Pass filter, the volume above the cutoff point is reduced by 12db per octave. A 4 pole filter would cut at 24db per octave, and so have a stronger sounding filtering effect.
Common filter types
These are essentially the same as an audio rate oscillator, but move slow enough to be useful for controlling other modules. Most have the same common shapes as a VCO, triangle, square, saw, sine, etc, but exotic shapes are available. Some LFOs will have sync or VC in, allowing you to control the timing or speed from an external source. Usually LFOs will modulate from 0-5V or from -5-+5V and will require a VCA or attenuator if you want to use them for finer control.
Envelope and transient generators are used to generate triggered voltage control shapes. They are very similar in function to an LFO, except they will usually wait for a trigger or gate before starting, and often stop at the end of their cycle, though plenty of modules have modes to retrigger at the end of the cycle in a loop, essentially turning them into a flexible, syncable lfo. There are many types and configurations of envelope, but they usually follow the same nomenclature.
Typical configurations are ADSR, AR and ADR, but there are always exotic. Transient generators are a common term for AD or AR envelopes, often used for short, percussive sounds, hence the name. Note that unlike most other control voltage in eurorack, envelopes often go from 0-8Vs, because things. However, being eurorack, nothing is ever certain, and nothing is truly a standard.
There is a handy visual list of envelope shapes here: http://ornament-and-cri.me/envelopes/
These modules let you reduce the strength/volume/width of a signal, hence the name. Many attenuators also have an offset so you could, for example, take a 5V wide LFO, attenuate it to 1V and offset it by +1V so that it outputs a 1-2V range. An attenuverter is similar except that it can invert the polarity of a signal, so that at 12o'clock the signal will be 0V, and full anti-clockwise the signal will be inverted and attenuated versions anywhere inbetween. These functions are most useful for control voltage, but can be used for audio. For example you could mix an inverted version of an audio signal with the original and it should produce silence (though only in a perfect system, which analog generally is not), and you can modify one or the other signal and the result would just be the difference of the two signals. Voltage controlled attenutators can perform the same functions as a VCA. Voltage controlled attenuverters can perform the same function as a Ring Modulator or Waveshaper.
As the name suggests. It'll mix signals together. Commonly for audio, but most mixers will let you mix control voltage together as well. The only rule is, many signals come in, only one come out! Mixers are generally manually controlled with knobs, but many VCA modules also double as mixers and allow you to control the mix with external signals if that is something you want to do. Complex mixers will let you send signals to auxiliary outputs or fx loops, just like a typical desktop mixer but are usually pretty expensive in the modular world.
Mults are kind of the opposite of a mixer. They'll take one signal and split it into multiple copies of the signal (hence the name). They generally have two configurations, Passive or Buffered.
Passive mults don't use any power and are pretty much the same as just an audio Y cable. They are fine in most circumstances, but because of the nature of voltage when you use a passive mult, you are splitting the voltage between all of the outputs. This usually results in a lower signal which is fine for things like audio, gates and triggers, but will result in weirdness for things like pitch CV and other level sensitive CV. This is where buffered multiples come in, these are powered and use op amps to make sure the copies are identical to the input, perfect for sending a single pitch to multiple VCOs.
Because of how they are built, there is nothing stopping you from reversing the use of a passive mult and sending multiple signals in to one (or more) outs, but be warned that multiplying voltages together in a non-buffered way can cause high voltage spikes that some modules may not be prepared to handle! Often it won't be a problem with most designs, but it's totally at your own risk. It's not hard to imagine blowing an input's opamp because you exceeded the rail voltage of 12V.
These are essentially pulse wave LFOs (sometimes very very short pulses), but are meant to keep time in a modular system. You can often just use a square wave out from an LFO to do the same thing. The main difference is that they will often output short trigger style pulses to make them useful for triggering things that might behave differently when using a full 50% duty cycle square wave.
These will take a clock signal (or other relatively constant signal) and multiple or divide the number of incoming repetitions by the amount based on the function. Commonly there will be one input which will spit out 2/4/8/16/32 etc multiples or divisions of the original timing, or will output a trigger on the 2nd, 3rd, 4th, inputs. These are incredibly useful for creating rhythmically synchronised variations in your modular. Clever use of these modules can let you create interesting drum/rhythmic patterns.
Sequencers at their most basic will output a sequence of voltages in time to an input clock signal. Sequencers can be super simple, or incredibly complex in the modular world. The two basic configurations are CV/pitch and trigger/gate seqs. Obviously CV seqs are usually used to create melodic sequences, while trigger seqs are usually used for percussion. I won't get into the huge number of common functions and terminology available in this space, but suffice to say if you want to start writing songs with your modular you will want to explore some of the options availabe in this space.
Frequency Modulation or FM is the term for when you modulate the pitch of an audio rate oscillator with another audio rate oscillator. It generally creates metallic warbles and usually sounds most harmonic when the modulator is a divisible ratio of the modulated signal.
Classically speaking this is done with multiple sine waves, but there is nothing stopping you from using anything at your disposal. You can create interesting configurations as you add additional oscillators in series (modulators modulating modulators), or feed back the output of oscillators back into the chain.
Yamaha made these kinds of sounds famous in their DX-7 synth, but it also features heavily in the chip sounds of arcade games and sega consoles, many of who also had sound chips designed by Yamaha such as the YM2612.
Amplitude modulation as you can probably guess is modulation of the volume of a signal by another. Once you modulate the amplitude of a signal past that magic 20Hz audible range you start getting interesting overtones added to the original signal. You can create this by patching two VCOs into a VCA, one the audio input, and the other the VC input.
Ring modulation is very similar to AM, except that the two signals are multiplied together resulting in both negative and positive values, which result is a sound that is closer to (an often inharmonic) combination of both sounds rather than one modulated by the other. There are dedicated Ring Modulators modules to be found, but you can patch this up manually if you have a voltage controllable attenuverter.
Refers to the modulation of the ratio of a square/pulse wave from 0-50%. A square wave is a perfect 50% pulse, whereas 0% produces silence/control voltage at max or minimum voltage of the oscillator. Anywhere inbetween produces a nasal, thin sounding square. However, slowly sweeping this range creates a rich, deep classic bass sound. Modulations in the upper ranges can produce other interesting overtones and timbres.
Some typical or interesting patch examples to get you started