like when you say tu and tú
What is the difference between tú y tu?
Tu refers to possession (your), as in
Es tu libro (It's your book)
Tú is a pronoun that means you
Tú eres inteligente (You are inteligent)
As Izanoni1 says:
tú is a personal or subject pronoun
tu is a short form possessive adjective
also to note, becareful, as many people such as myself will type "tu" without the accent mark even though I mean "tú" with the accent mark, as I am lazy and have US keyboard layout.