Cómo estás: This means "How are you?"
Cómo eres: This is almost asking you "What are you like?"
I remember this clearly because my Spanish 2 teacher would makes all the students in her class either answer to "¿Cómo estás? O ¿Cómo eres?" And we'd have to reply accordingly.
As to what the other responder is trying to get at is how you conjugate the verb; whether that be in the Uds. or Tú conjugation.
