As well as the many materials knitting needles might be made of, there are also many other factors you can consider when choosing which needles to use.
Straight needles are the ones you see most often. These are perfect for most pieces, but once you have a large number of stitches or you are working a circular piece, you will be wanting a different needle.
Circular needles can be used instead of straight needles if your piece has a large number of stitches. Simply use each end as if it is a separate needle, and your work falls along the cable in the middle. I find circular needles useful for travelling, as theres less chance of misplacing one needle.Seamless work must be knitted in a circle and this requires a circular needle. Simply choose a needle that has the needed length of cable, cast on, and instead of turning the work at the end of the row, join it at the end and keep on knitting. Knit & Yarn can supply interchangeable knitting needles. These allow you to swap cables and tips for a limitless combination of sizes and lengths. The detachable cable also makes it possible to leave your stitches, without needing a stitch holder. Stocking stitch will require you to knit every row, and garter stitch will require you to knit one row and purl the next.
DPN or double-pointed needles come in most sizes, in sets of 4 - 6. These look more complicated than a circular needle but they're quite simple to use. Sometimes a circle is too small for a circular needle (baby hats for example), and this is when your dpns are really useful. simply cast on as normal, distribute the stitches evenly between you rneedles, keeping one spare to work on, then knit as normal on two needles, allowing the unused ones to rest until you knit up to them.
The final point to consider when choosing your needles, is a fairly modern one. until recently, all needles were round, but it is possible now, to buy cubic knitting needles, which are square shaped. These provide extra grip, and can give a more even stitch, so if you have difficulty with your tension, it might be well worth considering these as an option.