It's and Its

This is an exception to the usual rules of apostrophe use.  Its means "belonging to it."  The usual apostrophe is omitted.  It's always means "it is."  With almost any other word, an apostrophe and "s” can indicate possession.

Example: It's amazing that a dog will chase its own tail.  

Example: The company needs to get its books in order before it's too late.