what is the difference

[Thomas]

Yeah, you guys is a way of saying "you" but making sure that it's plural. Even though we don't have the tu and vous of French, sometimes there is a need to indicate a plural "you". So we say...

you guys
you all
y'all (not used everywhere, only in certain dialects -- thought of as Southern by most people, but it is found outside the South, too)

Typically, "a guy" is "un mec" or as Freddyben put it, "un gar", -- however, in this case, it simply indicates a plural. It can even be used to a group of people that is entirely female (I just used it maybe 20 minutes ago to two girls to ask "You guys having fun?")

[Ben54]

Merci à Freddyben et Thomas pour ces précisions...