* Informal: Πού είσαι; (Pou eísai?) This is used with friends and family.
* Formal: Πού βρίσκεστε; (Pou vriskeste?) This is used with strangers or people of higher social standing.
Both start with "Πού" (Pou) meaning "where". The difference lies in the verb "to be":
* είσαι (eísai) is the informal singular "you are"
* βρίσκεστε (vriskeste) is the formal singular/plural "you are" (literally "you find yourselves").