The bastion of analog computing. "op-amp" is short for "operational amplifier."
An op-amp is a circuit whose output is the sign of the difference of the two voltage levels applied to the inputs.
Here's a schematic diagram:
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As I said, a lone op-amp will only output the sign of the difference between the inputs. The sign is determined by subtracting the voltage level at the "-" input from the "+" input. If the sign is positive:
Vout = V+
If the sign is negative:
Vout = V-
If you want to use an op-amp to amplify by a certain factor, you need to use negative feedback.