Γεια/Γεια σου/ (Ghia/Ghia sou) : Informal way of greeting someone
Γεια σας :(Ghia sas), formal way of greeting just one person / greeting more than one person
/Χαίρετε/Γεια Χαρά:(Herete/Ghia Hara) applies to both cases (nice way to say hello, a bit impersonal)

GHIA

This greeting covers a wide range of translations in Greek: If the person you're greeting is owed respect, then the translation is "Γειά σας" or "καλημέρα σας" (the latter if it's morning) or "καλησπέρα σας" (this last if it's evening. If the greeting is informal then the respective expressions are "Γειά" or "Γειά σου", plain "καλημέρα" or "καλησπέρα". If the greeting is an informal one to somebody who's entering the room the translation is "καλώς τον/την/τες/τους" (male sing/female sing/female pl/male pl). If you're simply answering the ringing telephone with 'hello'(?), the correct translation is "Παρακαλώ" (?) or "Eμπρός" (?)

I agree with all the above, with the only difference that the correct spelling for "γεια" is without an accent, whereas another way of saying "hello" when answering the phone is "Λέγετε".
More slang uses of the word include: "γεια χαραντάν", "τσάγια" (mainly in Thessaloniki, coined from "τσάο" and "γεια"), etc.

The ancient greek greeting, meaning 'have joy'. Plural: hairete (cherete).

English: HELLO
Greek: ΓΕΙΑ ΣΟΥ (informal, friendly singular)
ΓΕΙΑ ΣΑΣ (plural or polite by the use of plural)
ΧΑΙΡΕΤΑΙ (formal, polite always plural=polite)
ΟΡΙΣΤΕ (when answering the phone, polite as you do not know who is on the other side of the line).