(1) the expression used when addressing either someone you know well or your significant other. (2) sometimes used as a word to start stories with inflection. (3) a way to express anger at someone you don't know.
(1) dude! how's it going? (or) i really like you, dude!
(2) dude, so there was this guy...
(3) DUDE! GIVE ME BACK MY PURSE!