Appearing in the 60s, the earliest voltage-controlled analog synthesizers were modular synthesizers. Unlike common “all-in-one” instruments, all sound sources and modifiers contained in modular synth are separate building blocks, aka "modules," that can be patched together in an infinite variety of configurations for unparalleled flexibility. Typical basic modules include oscillators, filters, and amplifiers. By themselves these modules might not do anything interesting, but together they form the voice of a musical instrument.
Modular synthesizers have no rules. Modules can be patched in virtually any configuration inspiring all sorts of creative sound experimentation. Eventually, large modular systems were replaced with smaller, pre-wired portable instruments. These were convenient and musical, but their fixed signal routing greatly diminished the experimental nature inherent to modular synthesis.
In recent years, people have rediscovered the sonic possibilities of modular synthesis, resulting in a tremendous resurgence. With hundreds of different companies building hardware modules, systems can be built and customized in an infinite variety of ways.