Developed as early as 1963 Modular Synthesizers a system consisting of 2 separate modules specialized and connected together with patch chords.
One of the first creations of this portable synthesizer is the Minimoog which was a big hit n 1969 under semi-modular synths. Later on, it was developed as an instrument and the name Moog became synonymous to synthesizers.
Now the modern modular synthesizers are broken down into modules and can be connected thru the inputs and outputs using the following signals CV (control voltage) and Gate. CV controlling the sound source while the Gate triggers the events with the use of on and off.
You can read more here: http://music.tutsplus.com/articles/what-is-modular-synthesis–cms-26896