A double pointed knitting needle is exactly what it sounds like — a knitting needle with points at both ends. They tend to come in larger numbers than straight, single pointed needles, usually four or six to a pack. Most materials used for making straight needles are obviously suitable for making double pointed needles. Aluminum is, unfortunately, by far the most common these days.

Knitters seem to have a love-hate relationship with DPNs (as they are abbreviated). They drop stitches far too easily (especially when made of aluminum), since there is nothing keeping the stitches on the needle. On the other hand, there is some knitting that is traditionally done on DPNs, like Shetland lace. They're good for doing small, straight knitting, and for making socks. Though, of course, you can make socks on circular needles too.

Casting on to DPNs for knitting in the round

1. Cast on the required number of stitches, using a straight, single pointed needle of the same size. If the number of stitches is small, you can cast on to the first DPN and save yourself some trouble.
2. Divide the stitches up evenly among your double points, slipping them from the single point in succession. You should have one long chain of double pointed needles hanging precariously by the tension in the cast on row. Don't let any stitches drop!
3. Now, take the first and the last needles and proceed to knit across the gap in the cast-on row, using a spare DPN as the working needle.
4. When you meet a gap between one needle and the next, be sure to tighten down the stitch that falls in the gap. Try to distribute the stitches so that the gaps fall on knits, I find it's easier to tighten those down.
5. When worked needle is empty, switch hands and make it the new working needle. The effect is very similar to that of juggling.

If you don't tighten down the stitches in the joins, you'll end up with "ladders" — a gap in the work that looks like a dropped stitch that has fallen all the way down.

yclept, bewilderbeast, and iamkaym all helped educate this newb on the finer points of double pointed knitting needles. Go ching! their nodes, not mine! ^_^;;