In some cases, heterozygotes have greater reproductive success than homozygotes. this effect tends to preserve variation in a gene pool and is known as a heterozygous advantage.
Overdominance is a specific case of heterozygotes advantage when a heterozygote produces a phenotype which is better adapted than phenotype of the homozygote. The example is sickle cell anaemia (homozygotes will have no protection from malaria while heterozygotes have a partial resistance to malaria).