what's the difference between the different yarn weights?
i can get the obvious ones, like sock yarn being for socks, and lace weight being for lace, but what about the general ones like DK, worsted, sport, carrying, etc?

It has to do with how thick they are, and therefore how many stitches per inch you get out of them. The 'regular' weight that you'd see the most of is worsted weight, about 4-5 stitches per inch.

Dk is about half the thickness, sport a little thinner.