Salutations

Dzień dobry - Good day (formal) - can be used at any time of day

Dobry wieczór - Good evening

Dobranoc - Goodnight 

Cześć - Hello

Hej - hey

Witam - Hello - A bit more formal than cześć - used when you are not familiar with the person / people you are greeting.

Witaj - Hello - Used when you are greeting one person you are familiar with.

Witajcie - used when greeting multiple people you are familiar with.

Serwus - Hello - colloquial

Do widzenia - Good bye (formal)

Do zobaczenia - See you later

Do zobaczenia w piątek - See you on Friday

Cześć - Bye

Pa - Bye

Pa pa - Bye bye

Do jutro -  See you tomorrow

Spadam - I'm off (I'm falling) see ya

Uciekam - I'm off (running) see ya

Na razie - So long (for now)

Nara - a contraction of na razie, considered impolite by some

Żegnam - A formal and possibly final goodbye or farewell - at a funeral, priest would say:

- Żegnamy dzisiaj...  "Today we say goodbye..."