Differential amplifier and op-amp
A differential amplifier traditionally has differential outputs: -



Opamps almost without exclusion have a single output. However, over the years the distinction has become a bit cloudy. For instance, this is also called a differential amplifier: -



Basically it's an op-amp with a differential input configuration.

Then there is the difference amplifier that I would say is the more correct way to describe the op-amp circuit above. There is also the Instrumentation amplifier (really just a difference amplifier): -