An electronic musical instrument requires the physical sensing of the musician's movements and gestures. This is what a class of devices called controllers does. Over the years, many kinds of controllers have been created to equip a composer or performer with an instrument they can play.
The direct manipulation of the controllers. The composer uses cables (patch cords), knobs and switches to trigger sounds. This was how the original RCA, Moog and Buchla synthesizers worked. They did not have piano keyboards. This was because the early synthesizers were not viewed as performance instruments. They were used with tape recorders in the context of an electronic music studio, and the music was put together piece by piece by editing on tape. Synthesizers are still widely available today as rack-mountable components without a keyboard. These sound modules can be operated by the musician using MIDI signals from the control keyboard or via software from the computer.
Keyboard controllers. Keyboards are the most common form of controller used in synthesizers. Keyboards today are polyphonic and can play more than one note at the same time - but this was not always the case. Commercially available voltage-controlled analog synthesizers, available in the late 60's and early 70's, could only play one note at a time: the highest note played on the keyboard at a given moment. Each key represented a different voltage value. The early synthesizers were not artist-friendly. Even the simplest chord had to be created through a multi-track recording.
Keyboards are usually touch-sensitive, so the harder someone hits the keys, the louder the sound becomes. Another common feature of electronic keyboards are the control wheels for pitch-bending or for adding a rippling tremolo effect.