Double-pointed needles in action.(c) Sarah E. White, licensed to About.com, Inc.
Double-pointed needles are knitting needles that have points at both ends, rather than having a pointed end and an end with a stopper that keeps the stitches on the needle.
Double-pointed needles are popular for sock knitting and other small projects knit in the round because they allow you to seamlessly knit something with a small diameter.
They come in most of the same sizes and materials that other needles do and can be purchased in packs of four or five. Some patterns will call for four needles while some call for five, but you can pretty easily compensate for using a different number of needles if you understand sock anatomy.