opamp as inverting amplifier
Each opamp is a high-gain differential amplifier. Therefore, the unit can do nothing else than to amplify the voltage difference between both input nodes - independent if one of the input nodes is grounded or not. This voltage difference is in the micro-volt range and does not allow real applications.

However, because of the large open-loop gain (Aol) of the opamp unit we always use negative signal feedback. For simple amplifier purposes the feedback network consists of resistors only. Referred to the applied signal voltage, the so-called closed-loop gain Acl is reduced correspondingly and the applied input voltage now can assume realistic values in the millivolt or even volt range. 

However, the voltage difference directly BETWEEN the opamps input nodes is still in the micro-volt range. During calculation of opamp amplifiers (with feedback) this tiny voltage can be neglected in comparison with the two other voltages within the circuit (applied signal input voltage, opamp output voltage). This approach leads to the concept of "virtual ground" (for inverting opamp applications).