It's similar to the word "really", exxcept it's much more emphatic. In general, i think that it is mostly used to emphasize something someone needs or wants, in fact, usually only with the two words "want" and "need" "I really like this" "You desperately need a cookbook" "I desperately wanted to go there" "I need you desperately." (Less often used this way. generally only comes before the verb)