I don’t like to link to other blogs or articles for I am just a lame copier, but I’d like to point my select few readers to Enter The Unkown, the weblog of Kaja Fumei, where he will point out some Algorithms a Programmer should know. First one in the list are the Hash Tables.
You don’t need to know the stuff behind these kind of features a language library exposes; but it helps a lot if you know how it works and what the weak and strong points are.