The double pointed needle is probably one of the most intimidating knitting tools. The intimidating part about DPN's isn't the individual needles, they look a lot like big toothpicks. The part that gets most people about DPNS is that they are used in groups. I know whenever I am using DPNs I get strange looks, because I'm using 5 needles at one time. To most people, it seems like there are too many things going on if five needles are in one project at a time. The thing about it is that it really isn't hard to use DPN's, the secret is to make sure that the needles you aren't using at a particular moment don't slip out of your stitches. The best thing to do to ensure that the stitches don't slip off of the needles is to use needles made from a rough material. My favorite double pointed needles are made of bamboo. These needles aren't as smooth as aluminum needles, but they are still smooth enough to slide through yarn without snagging it.
So, what are DPN's used for? Double pointed needles are used to make cylindrical garments, such as socks and sleeves. DPNs are useful for those instances when a circular needle is too big. Yes, there is another way, probaby more than one way, but the one that comes to mind right now is the magic loop method, which allows knitters to use long circular needles to knit things that are smaller in diameter than the length of the circular needle, if that makes any sense. I have not gotten completely comfortable with the Magic loop method, but when I do feel a little more proficient at it, I will prepare a tutorial on it and share it with you.