The Difference between Tú and Usted

Tú and usted both  mean you.

Use Tú when talking in an informal setting (family, friends, one-night stands).
Use Usted when talking in a formal setting or when you need to suck up (doctors, teachers, prison wardens).

To remember which you goes with who, I picture this:

Tú is wearing a baseball hat because she is informal. Meanwhile, the formal Usted is looking down his nose at  you like some kind of Orwellian  big brother.