"witaj" is more formal kind of greetings. "Cześć" is a form using amog friends.

Hello ;) "Cześć" is a word to say "hi", but just to close friends, or people you are introduced to, but must be in similar age. More safe in this formal situation is to say "Witam". About "cześć", it's a word u say for beginning and for the end of meeting. "Witaj" it's more formal way to say "cześć", but only with a person you already know.

"Witaj Marek"- you'd use if Marek is your buddy and you greet him; "Cześć Marek" - like "Witaj Marek"; However, if you need to be more formal with a person (she/he is much older than you, for example) you can use: "Witam" (but not "Witaj"- because that would mean that's your buddy). "Witam" is used without a name.