Quote Originally Posted by Aeneas
when you want to show possession it should be written its'
when you want the contraction of "it is" it's
Sorry, Aeneas, "its'" does not exist in the English language. However, you're in good company because this little word (or contraction) is probably the most misused ever.

There are only two forms:
"its" is the possesive: The dog had fleas in its hair.
"it's" is the contraction of "it is": It's cold outside.

The easiest way to get it right is, whenever you use "it's", substitute "it is" and see if if still sounds right. If it doesn't, you probably meant "its".