Both mean "you" Tú is informal, and usted is formal.
Tú [verb] will end in "s" (as, es)
Usted [verb] will end in the same ending as the he/she/it form (a, e)
Use usted when talking to a boss or someone that is above you. Use tú when your talking to a family member or a friend. This is because tú is the informal version of you, usted is the formal version of you.