The bastion of analog computing. "opamp" is short for "operational amplifier."
An opamp 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:
V+

  
 
 
+ 
xxx >Vout
 
 
 
  

V
As I said, a lone opamp 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:
V_{out} = V+
If the sign is negative:
V_{out} = V
If you want to use an opamp to amplify by a certain factor, you need to use negative feedback.