Input is what a program responds too. On Unix most input comes from stdin, though it can come from other sources. Contrast with output.

Supplied by a device attached to a central processing unit that supplies data to the processor from an outside source including mass storage devices like tape, hard and zip disk. Although mass storage is usually termed combined input/output. Normally input devices are human user devices like mice and keyboards and digital cameras.
This usually feeds into memory where it is processed and sent to output.