"¿Qué tal?" is a very casual "how are you?" sort of like the English "What's up?"
It isn't really a phrase that you can literally translate over to English though. It's more of an idiom. The closest literal translation I could think of would be "What's it like?" ["tal" normally refers to "such"]
When using it in more complex sentences though just think of it as "how is." For example: ¿Qué tal el gato? [How is the cat?]
"¿Cómo está usted?" is more like "How do you do?" or "How are you doing?" It's formal.