You = The only person you are talking to ("You're late, Sam") You guys = Two or more people you are talking to ("You guys are terrific!")

You plural is used when you are speaking formally or informally. You guys (which I consider a Northern United States phrase) is used informally. In other words, you would not say to your bosses, "You guys need to get with the program." Sometimes you hear a very bad slang, "youse guys" When you watch American movies, notice whether the people who say this are in the Northern states or Southern states or out west. We say ya'll in the South which is very improper English. Ya'll come here! See ya'll later. You come here! See you later!

“You” is properly used as the second person singular or plural pronoun in the nominative or objective case. “You guys” is quite common everywhere these days, although it’s more likely to be used by the younger crowd. In the South “you all” (often pronounced y’all) is the traditional pronoun used in addressing two or more persons or referring to two or more persons, one of whom is addressed. It is quite proper and correct.