stands for Arithmetic/Logic Unit

This is the part that executes the computer's commands.
A command must be either a basic arithmetic operation:
+   -  *  /
or one of the logical comparisons:
>  <  =  not =.
Everything else has to be broken down into these few operations. Only one operation is done in each Machine Cycle.

The ALU can only do one thing at a time but can work very, very fast.