Quick answer
¿qué pasa?
Natural: what’s up? / what’s going on?
Literal: “what happens?”
How it is used
¡Hola! ¿Qué pasa?Hi! What’s up?
¿Qué pasa aquí?What’s going on here?
¿Qué te pasa?What’s wrong with you?
No pasa nada, tranquilo.Nothing’s wrong, relax.
Where it is said
As a greeting it is especially common in Spain; in Mexico you will hear <b>¿qué onda?</b> and in Argentina <b>¿qué hacés?</b>.
A mini dialogue
— ¿Qué pasa, Marta?— What’s up, Marta?
— Nada nuevo, aquí trabajando.— Nothing new, just working.
Watch out
¿qué pasa?
Adding a pronoun changes the meaning: ¿qué te pasa? asks what is wrong with you, so it is not a friendly hello.
Check yourself
Which version asks whether something is wrong with the person?