If you meet with your friends, you say them "Привет!" ("privet!"). Also this word is used with respect to one's parents. In case of greeting unfamiliar people or elder people, in Russia we use the word "Здравствуйте!" (pronounced as "zdrastvuitje", it means "I wish you good health"); this greeting may be reduced to "zdrastje", if you meet someone, who elder than you or of equal age, but you meet with him (her) often and know each other well.
Also we have male unofficial greeting "Здорово!" (accent falls on the second "o", "zdarova!"); Russian men use it with respect to each other (not to women!).