cha
Cha can be interchanged with the fallowing words:
Who
What
Where 
How 
Why

Basically it's the all in one question word.
"Cha doing?" (1. What are you doing? 2. How are you doing? Etc.)
"Cha Going?" (1. Where you going. 2. Who is going? Etc.)
"Cha?" (What?)