Operational amplifiers are linear devices that can perform mathematical operations like addition, subtraction, integration and differentiation. They have ideal characteristics such as infinite input impedance, zero output impedance, and infinite gain. Common op-amp circuits include the inverting amplifier, non-inverting amplifier, summing amplifier, differential amplifier, integrator, and differentiator. The integrator produces an output voltage proportional to the integral of the input over time, while the differentiator produces an output proportional to the rate of change of the input voltage.