Types of selection
Directional
The example of industrial melanism is an example of directional selection. In directional selection one extreme of the phenotype range is selected for, in this case dark moths over lighter ones.
Disruptive
This is when two or more extreme phenotypes are selected for. For example, if there is an area with dark trees covered in areas of light lichen, natural selection will favour two types of moth - those with lots of melanin, which will camouflage against the dark trees, but also the type with very little melanin, which will camouflage against the light coloured lichen.
Moths with intermediate melanism will be selected against as they will not be successfully camouflaged in either environment.
Stabilising
In stabilising selection an average phenotype is selected for and extremes of the phenotype are selected against.
In an area where trees were less dark, and where there was less light lichen, moths at either extreme of colouration would be less successfully camouflaged and would be selected against. Moths with intermediate melanism would be selected for.
Another common example of stabilising selection is clutch size in birds. Birds that lay too many eggs have an increased chance of losing offspring to starvation.
Birds which lay too few have a decreased chance of these birds surviving and passing their genes on. Nature favours clutch sizes of an intermediate number.