Balancing selection
Balancing selection refers to forms of natural selection which work to maintain genetic polymorphisms (or multiple alleles) within a population.   Balancing selection is in contrast to directional selection which favor a single allele. 
A balanced polymorphism is a situation in which balancing selection within a population is able to maintain stable frequencies of two or more phenotypic forms. Evidence for balancing selection can be found by increased levels of genetic variation between alleles or haplotypes in a species. Note that balancing selection will not always result in an observable phenotypic difference because the genotype may not be one-to-one with the phenotype.
There are several major mechanisms (which are not exclusive within any given population) by which natural selection preserves this variation and consequently may produce a balanced polymorphism.  The two most well studied are heterozygote advantage (overdominance) and frequency dependent selection.  A less well studied alternative is environmental heterogeneity.

# Heterozygote advantage
In heterozygote advantage, an individual who is heterozygous at a particular gene locus has a greater fitness than a homozygous individual. 
A well-studied case of heterozygote advantage is that of sickle cell anemia.  This can be seen in human populations with the locus for a certain protein present in hemoglobin (an important component in blood). Individuals who are homozygous for the recessive allele at this locus are inflicted with sickle-cell disease, a disease in which red blood cells are grossly misshapen and which often results in a reduced lifespan.
An individual heterozygous at this locus will not suffer from sickle-cell disease but because of slightly irregularly shaped blood cells they are resistant to malaria. This resistance is favored by natural selection in tropical regions where malaria (a common and deadly sickness caused by the protozoan parasite Plasmodium falciparum) is present and so the heterozygote has an evolutionary edge. It is in this way that natural selection preserves stable frequencies of both the heterozygote and the homozygote dominant phenotypes.

# Frequency-dependent selection
The second important mechanism by which natural selection can preserve two or more phenotypic forms is known as frequency-dependent selection. Frequency-dependent selection is a form of selection in which the relative fitness of a specific phenotype declines if the frequency of that phenotype becomes too high. An example of this type of selection is between parasites and their hosts. An example follows: suppose that a certain parasite can recognize one of two receptors in its host, receptor Alpha or receptor Beta, if many parasites with receptor Alpha exist then hosts with receptor Beta will be selected for, and this will subsequently increase the selective pressure on parasites which use receptor Beta and this relationship will continue rocking back and forth.
Frequency-dependent selection has been observed in the banding and colour polymorphism in the European land snails, Cepaea nemoralis, where thrushes preferentially predate the most common morph. Frequency-dependent selection also appears in the form of mate preference, a type of sexual selection.

# Environmental heterogeneity
In the case of environmental heterogeneity, when the environment conditions fluctuate, it may give the normally selected-against organism some form of advantage. An example would be the Biston betularia peppered moth, which has both dark and white polymorphic states. During snowfall, when the fields are covered with snow, it is more likely that the white forms are selectively favoured. The balance is tilted in the other direction when the snow disappears.