Your and you're; its and it's

"Thank you."
"Your welcome." Aaiiiieeeee! Please kill me now.

This is an error that seems to be an epidemic at all levels of society, especially on e-mails.

• "You're" is a contraction of "you are."
• "Your" modifies nouns, and in "your house."
• "It's" is a contraction of "it is," as in, "It's a beautiful morning."
• "Its" is used to modify nouns, as in, "The cat uses its litter box."
  
  These and other common confusions are listed in Harbrace's Glossary of Usage, (G-1 through G-11, near the back, marked usgl on the upper right corner of the right-hand pages.