"Dude" can be a friend or just someone you see on the street, as long as they are a man. You do not use "dude" in business meetings or other formal settings - just to talk to friends. Some examples: "Who was that dude you were talking to last night?" - This is the same as saying, "Who was that guy [you did not know] you were talking to last night?" Some people also say "dude" to place emphasis on things, like, "Dude, my car got stolen!" or "Dude, can you believe that?" Here, the use of "dude" is more like saying, "hey, you" to get people to pay more attention to what you're saying. You should only use "dude" with your friends, though, since it is very informal.

In the UK, "dude" is only used by the intellectually vacant who watch too much American TV.