Here's a bit of an explanation which might help: There are 3 main ways to size needles in the English speaking part of the world, all of which designate the thickness of the needle, NOT the length.
1. American sizes, which start with the smallest numbers for the thinnest needles, and the highest for the thickest.
2. British sizes, which go in reverse-- the higher the number, the thinner the needle.
3. Metric sizes, which are really the best way to go IMO, because they indicate the true measurment of the thickness of the needle.
There are also 3 basic types of needles.
1. Single point. These are straight and come in pairs-- each needle has a point on one end and a stopper of some sort on the other. They come in a variety of lengths, the most common being 10" and 14". You can knit flat pieces with these.
2. Double point. These come in sets of 4, 5 or 6, and each needle has points at both ends with no stoppers. They allow you to knit in a tube.
3. Circulars. These are like a pair of short single points, which are connected by a cable. These also allow you to knit in a tube, although you can treat them as if they are 2 needles and knit flat pieces, as well.
Each of these 3 types can come in any size thickness of the needles. So you could buy, say size 10 American (which are 6mm in metric) and get them in single points, double points or circular.
And they come in metal, glass, plastic, various woods. . .but that's a whole other post-- or 5:-).
Hope this helps!