Modular synthesizers were the first type of electronic sound synthesizer
created, and they are still unique in many ways. Modular synthesizers are
constructed of many building blocks of sound, called "modules".
The modules consist of various analog components that either create or alter
sound waves. There are two types of signals present in a modular system.
The first type are "control voltages". These are signals of various
voltage that tell the modules what to do. Control voltages could be used to
control the pitch of the note, the gate (when the note starts and ends),
or any number of parameters such as filter cutoff or LFO frequency. The
other type of signals are "audio signals". They consist of the actual
sound waves, and can be listened to (as opposed to control voltages, which
would be silent). Modules are connected with patch cables that route the
sound output or control output of the modules to and from each other.
Most modular synthesizers are monophonic, meaning they can only make one
note at a time. Technically, one could make a polyphonic modular, but
it would be enormous, and programming a single patch could take days.
Modulars are traditionally controlled by a keyboard, but the vast array
of knobs and sliders are what give the modular its power. Modulars are
more tweakable than traditional analog synths. The patch cables allow
the programmer to do wild and crazy things with sound outputs-- one can alter
any signal with any module in a modular system. The classic image of a modular
system is one of a scientist in a lab coat. He (or she) stands in front
of a veritable wall of knobs, moving wires in and out of their sockets.
The first practical and widely used modular synthesizer was the Moog modular,
constructed by Robert (Bob) Moog. Moog built his first modular systems in
the early 70s, and they were quickly adopted by any artist who wanted new
sounds-- everyone from mainstream rock bands to avant garde composers
and sound designers. The same type of people are still drawn to modulars today.
Many niche manufacturers such as Doepfer, Analogue Systems, Wiard, and
several others are still making full fledged analog modulars. Much more popular,
however, are software synthesizers that emulate modular systems (such as Reaktor
and VAZ) and hardware units, namely the Nord Modular.
A short list of analog modules
VCO -- Voltage Controlled Oscillator
VCF -- Voltage Controlled Filter
VCA -- Voltage Controlled Amplifier
LFO -- Low Frequency Oscillator
ADSR -- Envelope
S&H -- Sample and Hold
Ring Modulator
Noise Generator
Mixer
MIDI to CV Converter (Modern units only)
There are many others, but these are the most common.