In a nutshell

These are the things that drive me mad every single day…wish I could send Grammar Girl to all the offenders!

You're Versus Your
"You're" means "you are." "Your" is the possessive form of "you."

It's Versus Its
"It's " means "it is" or "it has." "Its" is the possessive form of "it."

They're Versus Their Versus There
"They're" means "they are." "Their" is the possessive form of "they," and "there" is usually a place.