These two words are often misused and confused in written English. "It's" means "it is" while "its" is the possessive pronoun that means something belongs to something else. When in doubt, replace the word "its" with "it is" to derive the correct meaning. For example, you would not say "It is tail." No, the right form would be "Its tail." Otherwise, it would look like "It's tail" and that would be grammatically incorrect.