Operational amplifier is an amplifier which amplifies the input signal when we pass a low voltage signal .It has two terminals inverting and non inverting.
It is an active device because it uses supply to work.
consider an open loop configuration ( when there is no feedback):
The gain is infinite (according to ideal operational amplifier condition) but practically it is not true. When we provide a less voltage(~1v)
Then the max voltage which we get at output is the supply voltage provided by us to the operational amplifier (op-amp) but the output waveform is not similar to the input signal (amplified) but in case of negative feedback it provides the output waveform similar to the input waveform (truely amplified )