The confusion with these words seem to be the most common problem on this forum, based on how so many members correct other members when they use the wrong one. So here's a helpful web page. http://www.wikihow.com/Use-There,-Their-and-They%27re
There, describes a destination: "Look, it's over there!", Their describes someones' possesion: "Their breath was garlic, in it's smell" & They're describes a group are _____: "They're fearsome beasts!".
Yeah the shortening of "they are" to "they're" creates more confusion. But stuff like this is what makes English the most annoying language in the world and paradoxically the most interesting.