Originally Posted by Tevildo
Toric lenses, to use the technical term, are usually heavier at the bottom than the top, so that they take up the correct alignment through gravity.
Gravity doesn't play a significant role in most modern lenses anymore. The more normal mechanism relies on blinking to keep them oriented. The top and bottom edges are shaped so that the motion of either the top eylid alone or both working together, depending upon the specific lens design, will cause a drag force which the lens rotates into, much like a weather vane always turns to face into the wind.
There's another type which can be used in the case of severe corneal astigmatism, which shapes the inner curvature of the lens so that the lens naturally sits on the cornea in the correct orientation, the way that would happen if you sat a football into an oval indentation.