---"¿Qué tal?" -> it sound more like the typical phrase you say like "hey", "okay", "hi".
---"¿Qué tal estás?" -> it is a bit more serious, just a bit. In this one you show more interest about how he/she is instead of just saying something greeting
When you answer someone's "¿qué tal?" or "¿qué tal estás?" you can ask him/her as well and easier by asking "¿y tú?".
Anyways you can say too "¿qué pasaaaaaaaa?". It is like the Spanish "wazzup!!!"