Opamp: One differential input pair, one single ended output, very high open loop gain (practically infinite at DC)
Differential amps: In a technical sense, Opamps are differential amplifiers. But when an amplifier is referred to as "differential" it's usually referring to differential outputs, which typical opamps don't have. Sometimes these have internal feedback resistors, in order to improve CMRR.
Instrumentation amps: One differential input pair, but the structure is normally more sophisticated and gives much better DC characteristics (offset and bias currents, etc) and better CMRR than standard op amps. Also their gain is usually not determined by external feedback resistors, but rather by a single external resistor.