Здравствуйте - a polite, formal greeting. (Здравствуй - less formal, but still very polite, used only for greeting a single person, while Здравствуйте can be used for greeting both a single person, and a group of people.)).
Добрый день! Доброе утро! Добрый вечер! - another version of a polite greeting, its usage depends on the time of a day.
Привет - the most widely used informal greeting. Can't be used as a formal greeting. (there's also the version like Приветик! - a bit girlish one))
Здорово! - an unceremonious form (widely used between close male friends).
Хэлло! Хай! - slang variants of informal greetings, came from English. They're used not very often as far as i can judge)