As the name implies it is an operational amplifier. It performs mathematical operations like addition,subtraction,log,antilog etc.. The main reason for OPAMPS used over transistors is that transistor can only amplify AC while OPAMPS can amplify AC and DC. You can get good amplifier gain in OPAMPS. The most commonly used OPAMPS are 741 and 324. IC741 is used in close loop configuration and LM324 in open loop configuration. i.e LM324 mainly used as comparator while 741 for amplification,addition etc...