Tu v.s usted
Tu is used for a causal way of talking, were as usted is used in a more formal way.
Tu you would basically use when you are talking to peers, friends, or anyone the same age as you. Usted is used when you talk to someone with an important title (say, the president) an adult, or someone older than you.
Here's an example between to friends-
Amy- "Hola! Como estas (tu)?
Ellen- "Hola! Moy Bien, gracias. Y tu?"
Amy- " Mas o menos, gracias."
Ellen- "Adios!"
Amy- "Adios!"
ENGLISH
Amy- " Hello! How are you doing?"
Ellen- Hello! Very good, thank you. And you?"
Amy- " Regular, thanks."
Ellen-" Good bye!"
Amy- " Goodbye!"
Now lets see an example of a more formal converstaion between a girl and her mothers friend:
Girl- " Hola, Como esta ( Ud.)?"
Woman- " Hola, Muy bien, gracias. Y tü?"
Girl- "Muy bien, gracias."
Woman- " Adios!"
Girl- " Adios senorita!"
ENGLISH
Girl- " Hello, how are you miss?"
Woman- " Very good, thank you. And you?"
Girl-" Very good, thank you."
Woman- " Goodbye!"
Girl-" Goodbye Ms.!"
for more info, here are some sites you can look a: http://www.quiz-buddy.com/Spanish-info/tu_vs_usted_in_Spanish.html
http://www.spanish.bz/usted.htm