An operational amplifier (op-amp) is an integrated circuit (IC) that amplifies the difference in voltage between two inputs.
*: Closed-loop gain
There are several ways to categorize op-amps. For example, they are categorized based on: 1) manufacturing process (bipolar and CMOS), 2) power supply type (single-supply and dual-supply), 3) input circuit (differential P-channel MOSFET pair, differential N-channel MOSFET pair, rail-to-rail input, etc.), and 4) electrical characteristics (low input offset, low current consumption, low noise, etc.). Op-amps may also be categorized based on their applications. For details, see the FAQ entry “What types of op-amps are available?”
The following documents also contain related information: