The difference between its and it's

This is the first of what I'll refer to as the 'apostrophe posts'.  There are three, specifically that I need to address, though I'm sure more will arise as I ponder it.

So lets start with the shortest one. Its or It's.
Its is the possessive form of the word It. Now this one may be a tiny bit more confusing because virtually every other word in the English language uses the 's to indicate the possessive. 
It's is the abbreviation of It is or It has.  The rule is that if you can replace the word with "it is" or "it has," use it's. Otherwise, it's always its.