What is Operational amplifier?

An operational amplifier is an integrated circuit with two inputs — the non-inverting input at potential $V_+$ and the inverting input at $V_-$ — and one output at potential $V_s$, powered by two supplies $\pm V_{\mathrm{cc}}$ (typically $\pm15\,\mathrm{V}$) that are usually left off the diagrams. It amplifies the difference $\epsilon = V_+ - V_-$:

with an open-loop gain $\mu$ of order $10^5$ and a saturation voltage $V_{\mathrm{sat}}$ slightly below $V_{\mathrm{cc}}$. When $\mu\abs\epsilon$ would exceed $V_{\mathrm{sat}}$, the output sticks at $+V_{\mathrm{sat}}$ or $-V_{\mathrm{sat}}$: the saturated regime.