In English we use "you" to talk to a friend, peer, family member, adult, teacher, boss, principal, group of friends, group of teachers, etc.  We use "you" whenever we address anyone, or any group of people.

In Spanish, however, there are four different "you's".  It's easy, just follow along.
 

• First of all there are two classifications of people: familiar and formal

familiar:  friends, family members, peers, etc.  In other words, these are people you are familiar with.

formal:  adults, teachers (usually), principals, bosses, etc.  In other words, these are people you don't know well, do not have an established friendship with, or are people you should show respect to.
 

• Second, look at the number of people: singular or plural.  This needs no explanation.

So here we go.  We now have 4 options:  familiar singular, familiar plural, formal singular, formal plural.  The following is how these are expressed in Spanish
 

• familiar singular (talking to a friend or a family member, etc.)=  tú

• familiar plural (talking to a group of friends, family members etc.)  This is tricky.  In most American countries ustedes (Uds. or Vds.) is used.  In Spain and in most literature, as well as informally in many American countries, vosotros or vosotras is used.

• formal singular (talking to your boss, principal, esteemed Spanish teacher, etc.) = usted (Ud. or Vd.)

• formal plural (talking to a group of people you should show respect to) = ustedes (Uds. or Vds.)