What is difference between pre tension and post tension precast girders of bridge?

A simple explanation:

PRE tensioned- cables are tensioned, and the concrete is cast around it. The tensioning is done BEFORE casting the concrete (PRE = before).

POST tensioned- cables are placed inside of ducts or are greased to prevent bonding to the concrete, the concrete is cast around the cables, and then the cables are tensioned after the concrete has reached a desired strength. The tensioning is done AFTER the concrete is cast (POST = after).