I'm an English teacher, which might explain some of my pet peeve in this area, but come on... it's not that hard!
it's = contraction of it is; its= possessive pronoun (the bird flapped its wings)
your = possessive form of you; you're = contraction of you are
there = refers to a place; their = indicates possession (their house); they're = contraction of they are
And these are just a few of the words that people constantly mix up and misuse. Okay. Vent over. Flame me if you want! : )