Many knitters love using circular knitting needles for a variety of different projects, even those that are worked flat. Circular needles have firm, pointed ends that are joined with a flexible cable.
Again, the needles come in a lot of different materials, and the cable length can vary widely, from tiny 9-inch circles for sock knitting to super-long 47-inchers for afghan knitting or working the magic loop method. Usually you'll find them sold in lengths of 16, 24, 32, 40 and 47 inches.
Within the genre of circular needles there are fixed circulars and interchangeable needles, which usually come in sets with a variety of needle ends and several cables that can be attached to each other to make needles of different sizes with cables of different lengths.
If you like working with circulars it will probably be worth it to you to buy a set (or two!) of interchangeables so you have a variety of needles on hand at one time.
Circular needles tend to be a little more comfortable to knit with because the cable holds the weight of your knitting, putting less strain on your body. The fact that the cable holds all the stitches makes it less likely your stitches will fall off the needle while you aren't working, and the more compact movements you can make when working with them make them ideal for travel knitting.
It can be difficult to use them for some projects such as small-diameter knitting unless you use the magic loop or knit with two circulars at a time.