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Special Effects in PA Systems
Usually a multi-effects unit - which may incorporate a range of effects - is a 19″ 1U box with knobs or buttons (and more often than not some kind of menu display) on the front.
Multi-effects units are generally delay-based (sometimes also including pitch-shifting algorithms)
A multi-effects unit may well also include some sound-processing functions (e.g. compression/expansion), but typically as well as reverb and delay effects it will be able to offer some or all of the following:
Artificial Double Tracking. Double-tracking - pioneered by Les Paul - was used on the vocal parts of some early Beatles recordings: basically you record the same part twice. ADT simulates this effect by adding a small and constantly varying delay (usually also coupled with very slight variations in pitch) to the original sound, and mixing the changed sound with the original sound. You sound as if you're singing the same part twice. ADT can be used to ‘thicken’ a vocal part.
Chorus is essentially a multi-layered (and sometimes more extreme) version of ADT. In moderation, it can be used to add texture to a sound (for example, it can make a six-string acoustic guitar sound a bit like a twelve-string). Overdone, it can make a voice or instrument sound like a tape-recording with bad wow and flutter being played underwater.
Pitch-shift is an effect that is sometimes used to produce a sub-octave effect (by generating a copy of the original sound and halving the frequency of the copy). Pitch-shifting by whole octaves (halving or doubling the frequency) can have some musical applications. Other shifts can be used to create artificial harmonies, but getting it right can be tricky, and getting it wrong can sound dreadful. Dedicated Harmonisers and Pitch-Correction units are based on the same principle, and are available both as plug-ins for some audio processing software, and as stand-alone units, and some of the better products can sound musical.
Ehnancers or Exciters work by creating high-frequency harmonics of the original sound, and mixing a proportion of the harmonics with the original sound. This effect is a form of distortion, and mimics the effect created by modest tape-saturation, or very modest overloading of valve preamplifiers. Paradoxically, it can make a sound seem cleaner and clearer. Some enhancers also add sub-harmonics (which adds body to the low frequency sound). In moderation, they can help a particular treated part (e.g. lead vocal) to stand out, or add apparent body and clarity to a mix. They can also help restore the elements of a sound that a cheap microphone or pickup fails to reproduce. However, the effect can quickly induce listening fatigue if overdone or used where it isn't needed, so if in doubt, don't.
Flange is another delay-based effect, and simulates the sound you get when you use reel-to-reel tape players to play two copies of the same sound at the same time, and slow one of the recordings down by pressing on the flange of the reel. The result is full of changing phase-relationships. This is a ‘special’ effect, not one to be used experimentally by the sound-engineer during live performances.
These are based on normal reverb algorithms, but gated reverbs end abruptly rather than gradually. They can work well with drums. Reverse reverbs gradually increase in level before they end (also abruptly). Both of these are special effects. Play with them in the house or the studio, but don't bring them on stage unless you know what you are doing.
If all else fails, read the manual!
For connection details, refer to the unit's manual and/or to the section on Mixers. Usually only a single send is required (even where the effects unit has stereo inputs), and this can be taken from any postfade auxiliary. However, some effects units now allow two mono effects (one on each channel) to be used simultaneously, and if you need to use this facility you will need one send and one return for each channel. For some stereo effects (e.g. reverb) a stereo return is required (either to dedicated auxiliary returns, or to a stereo channel or pair of mono channels), so you will need to configure your sends/returns to meet your own requirements.
Some (usually small-frame) mixers incorporate an onboard effects section, and if your mixer has its own effects you are unlikely to need an offboard unit unless you specifically want an effect it doesn't have (or simulates badly), or need to use more than one effect at the same time.
Otherwise, having some sort of multi-effects unit is certainly useful.
For live work, you want a unit that gives you a useful range of effects (any of those detailed above may be worth having), and which is easy to operate. Highly complex menus and total user control over hundreds of subtle parameters are not much use without total familiarity and/or pre-programming and/or midi control. If you mean to program your reverbs before the show goes on the road, go for a unit that allows you to set up and recall user patches easily.