What is the difference between Здравствуйте and Здравствуй?
Здравствуйте is formal, and Здравствуй only semi-formal. (Привет would be informal.)
yes, it is. When you address a group of people, it will be always здравствуйте.
the difference is the same as between вы (plural you or formal singular you) and ты (informal you). This is imperative, and you will probably see this later in your studies that there are two imperative forms for all verbs: singular/informal and plural/formal.
I want to add. It seems to me, that Здравствуйте (здравствуй) = hello. Привет = Hi. In Russian language "Здравствуйте" (literally) = I wish you health. In colloquial speech we often (75%) use a short form "Здрасте", which is nearer to "привет"