Here are some pics I found:
You can see the Aline hem curves and it has a slight flare at the sides but the straight skirt comes down straight on the edges and across the hem.
Another basic style is the circle skirt:
Of course you can make lots of variations on these basic styles... different types of waists, ruffles or lace at the hem, slits or pleats, godets, etc. With circle skirts you can do 1/4, 1/2, 3/4 or full circles to make them more or less full. But most of the time if you look pat the design details, a skirt will fall into one of these basic categories.
For example, here's a pencil skirt which is basically a straight skirt that has a more narrow hem width than the pic of the black skirt above....it also has a low rise waist and a ruffle at the hem and some pocket details: