|SET by itself displays current variables|
SET with a variable displays the variable
SET /A is for arithmetic expressions( rounds up to nearest integer )
SET /P allows you to set a variable equal to a line of input entered by the user
an excellent source for specifics would be typing set /? at the command prompt, or by referencing http://ss64.com/nt/set.html. If you have a specific question, be happy to help.
EDIT: Many Thanks for the fix Razor2.3