Estas and Esta are both from the verb "Estar"="To be"
Estas is the tú conjugation of the word, meaning "you are (familiar)"
Esta is the usted/3rd person conjugation of the word, meaning "you are (formal)" or "she/he is"
You would use "como estas" when asking a friend "how are you [doing]?" You would use "como esta" when asking a superior "how are you [doing]?" or when asking "how is he [doing]?"
Hola acazevedom: Even though that is sort of the same idea, it is not a true translation. "Hi, how are you doing?" would be : "Hola,¿ como le va?" (literally, Hi, how is it going for you?). So it all depends on whether or not you want a true translation or just the idea of it. Chau