Positive assortative mating is the tendency of like to mate with like; negative assortative mating follows the dictum that opposites attract.
Mating can be assortative with respect to a certain genotype (e.g. individuals with genotype AA tend to mate with other individuals of genotype AA) or phenotype (e.g. tall individuals mate with other tall individuals).
When positive, assortative mating may increase homozygosity, but when negative it increases heterozygosity.