Skinny jeans are tapered and straight legs are cut straight. A lot of it has to do with the legs of the person wearing them IMO, because straights can almost look like skinnies on someone with a thicker leg. But on someone with really narrow calves, straights can look as flared as a boot cut. Skinnies will always be more tapered and stacked at the ankle, though. I made you a picture in polyvore, because I'm bored. Skinnies are on the left and straights are on the right...
http://www.polyvore.com/cgi/set?id=445139