I'm still struggling with sweaters, frankly. I hate sewing up, but the armpits are tricky in the round.
Nanou1, circular needles look like this . There are two firm needles and a cord connecting them.
They are good for doing knitting in the round (hats etc), but also for flat knitting, particularly if you knit German-style, as they let you put a lot of the weight of the knitting on your left hand, so the right needle can move more freely. Because German-style knitting involves picking up the yarn with the right needle, rather than throwing it around the needle, you move your right needle around more. You really don't want extra weight on it, if you can avoid it.