For those of you reading this that are not familiar with knitting lingo, a few definitions:
Straight needles are what you probably think of as standard knitting needles: long and straight, with a point on one end, which is used for knitting, and a ball or disc or other sort of cap on the other end, which keeps stitches on the needle from falling off the back end. They generally come in sets of two.
Double-point needles, or DPNs, are like straight needles except that they have points on both ends. You can knit off of either end, which makes them kind of like straights only better (unless you have problems with stitches falling off the end you aren't using). They generally come in sets of four or five.
Circular needles, or circs, are a long flexible cord that ends in knitting needles (or two short knitting needles connected by a cord, depending on how you look at it). As with DPNs, you can knit off of either end, but the flexibility of the cord allows it to be used by itself to create a round item, and also allows it to hold more stitches than a straight needle. They generally come in sets of one.
Size note for needles: They can vary in diameter (the ones I use most commonly are between US0, which is 2mm, and US13, which is 9mm, but I have seen, or seen reference to, needles which are as thin as mechanical pencil leads and needles that are as thick as a tree branch) and in length.
Knitting Looms come in varying shapes -- generally round, oval, or rectangular, though I have seen other options -- and consist of a base that has pegs sticking up, each with a slight groove in it, and a separate knitting tool, which has a handle and then a thin straight bit with an L-shaped hook at the end. They create knitted fabric but use a different mechanism for getting there.
Size note for looms: Although there is no standardized system that I know of, looms can vary by the number of pegs (which to some extent constrains the size of item you make) and by the gauge (how thick the pegs are and how far apart they are placed). The knitting tool does not affect sizing.
Yarn needles are large sewing-type needles designed for use with yarn; the eye is big enough for most yarns to pass through. Often the tip will be blunt, although sharp-tipped yarn needles do exist and have a purpose. They disappear like socks in a dryer.
