Op-Amps stands for Operational Amplifier, a device usually manufactured as an Integrated Circuit (IC) . User don't need to know about the Itegrated Circuit inside in order to work with Op-Amps . All you need to know is that Op-Amps acts as an amplifier, it amplifies the difference of the 2 different voltages at input by a gain or amplification factor A

Output of an operational amplifier:

$V_{O}=A(V^{+}-V^{-})$

Ideally $A$ is assumed to be equal to infinity. However, in practical op-amps, it has a high value. Furthermore, the gain $A$ is a function of frequency.

Below is the most commonly used configuration of Op-Amps