For what applications are op-amps used?

An op-amp is an IC that amplifies the difference in voltage between two inputs.
Op-amps can be used for various applications, depending on the external components added.
In the most basic circuit, op-amps are used as voltage amplifiers, which can be broadly divided into noninverting and inverting amplifiers. Voltage followers (also simply called buffers) are a type of commonly used noninverting amplifiers. Op-amps are also used as differential amplifiers, integrator circuits, etc.

Op-amps are widely used for various applications. It is no exaggeration to say that op-amps are found in almost all electrical appliances. For example, op-amps amplify analog signals from various sensors in IoT-connected home appliances and measuring instruments.