In Spanish, usted and tú are both ways of saying you.
Usted is formal, and comes from the old respectful form of address Vuestra Merced, Your Grace. It's used with people you don't know very well, in business or commercial relationships, or with people who are older than you.
When you use usted the verb has the él and ella endings:
¿En qué trabaja usted?What do you do for a living? formal
¿Qué hace?What do you do? formal
Tú, on the other hand, is informal: it's used amongst friends, family members or when adults are speaking to children.
¿Cómo te llamas?What's your name? informal
¿Qué haces?What do you do? informal
The words for your also change.
su trabajo your job formal
tu trabajo your job informal
There's an increasingly widespread use of tú these days, but if you're not sure which one to use, you should take your cue from the native speakers.
