It's function is to perform integer and difference solutions to analog inputs. It has - and + inputs. By choosing capacitive or resistive feedback you can alter the signal as needed....
To oversimplify it the "operational amplifier" was originally designed to perform mathematical operations in electronic analog computers. The designer set the mathematical operation of each amplifier...
Usually output of an amplifier is a voltage ,...but in case of Operational Transconductance Amplifier ,Iout (current ) is the output. This feature, makes it useful for Electronic control of amplifier...
the slew rate of an op-amp represents the maximum rate at which the output can change , no matter how large an input signal is applied the slew rate of an op-amp is specified in V/s or V/mew second.