An allele is an alternative form of a gene (in diploids, one member of a pair) that is located at a specific position on a specific chromosome.

Diploid organisms, for example, humans, have paired homologous chromosomes in their somatic cells, and these contain two copies of each gene.

An organism in which the two copies of the gene are identical — that is, have the same allele — is called homozygous for that gene.

An organism which has two different alleles of the gene is called heterozygous.

Phenotypes (the expressed characteristics) associated with a certain allele can sometimes be dominant or recessive, but often they are neither.

A dominant phenotype will be expressed when at least one allele of its associated type is present, whereas a recessive phenotype will only be expressed when both alleles are of its associated type.
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However, there are exceptions to the way heterozygotes express themselves in the phenotype.