A gate is the lowest level logical (as opposed to analog) circuit element. It takes a limited number of inputs, performs some operation on them, and produces an output (sometimes, the output and its negation).
Typical gates are:
- NOT (also known as an 'inverter')
- NAND (AND with inverted output)
- NOT (similar)
The first gates were built out of vacuum tubes; these were replaced with discrete transistors. Larger logical elements, starting with flip-flops, but including such things as counters, shift registers, multiplexers, etc, were built out of gates.
The next step was the creation of integrated circuits; the first ones each held a number of gates, but quickly the larger elements were then built as ICs too.