Directional selection is a type of natural selection that favors one extreme phenotype over the mean or another extreme. This phenomenon is usually seen in environments that have changed over time. Changes in weather, climate, or food availability lead to directional selection.
Disruptive selection is a type of natural selection that selects against the average individual in a population. The make up of this type of population would show phenotypes of both extremes but have very few individuals in the middle. Disruptive selection is the rarest of the three types of natural selection.
Sexual selection is a "special case" of natural selection. Sexual selection acts on an organism's ability to obtain (often by any means necessary!) or successfully copulate with a mate. Selection makes many organisms go to extreme lengths for sex: peacocks (top left) maintain elaborate tails ...
The most common of the types of natural selection is stabilizing selection. In stabilizing selection, the median phenotype is the one selected for during natural selection. This does not skew the bell curve in any way. Instead, it makes the peak of the bell curve even higher than what would be considered normal.