In genetics, a dominance relationship refers to how the alleles for a locus interact to produce a phenotype.
There<unk> e three main kinds of dominance relationships:
Simple dominance or complete dominance (simple Mendelian inheritance) over a<unk>  trait
Incomplete dominance
Codominance
In general terms, a gene contributes to production or expression of some trait or character (a phene), e.g. the color of one's  iris. A locus is similar, but more loosely defined: since phenes<unk> e usually readily apparent but determining which genes contribute to them is a complicated process, loci<unk> e simply locations in the genome which<unk> e known to be directly related to expression of defined phenes. Most loci can eventually be assigned to specific genes, but there is no straightforward 1:1 relationship between loci and genes, and whereas a gene always encodes for a protein, a locus might be in a noncoding DNA sequence.
An allele (or allelic variant) is any of the versions of some genetic locus that might exist in a population. Regarding the iris example, there exists an allele of the locus called EYCL3[verification needed] which causes the eye to be some shade of brown from lots of eumelanins, and another that causes few eumelanin to be produced, resulting in an iris that is blue, gray or green. Since you<unk> e diploid, you have two copies of the locus, one inherited from your father and the other from your mother. Thus, you can be homozygous in that locus - having one of the two allelic variants twice - or heterozygous, with one copy of each variant.
Brown eye color is the dominant trait in the iris example, whereas non-brown eye color is the<unk>  trait; often, non-brown is called simply "blue" due to the rarity of green and grey eyes, but this is technically incorrect (see next paragraph). If one or both of your EYCL3 loci carry the allele for the dominant trait, this trait  - brown eyes - will be expressed. The "brown eyes" allele induces copious eumelanin production in the iris, whereas the "non-brown" allele causes the production of only small amounts of eumelanins. Therefore, the<unk>  trait in this example is simply overwhelmed by the dominant trait.
To have green, blue or grey eyes, both copies of the EYCL3 locus must carry the<unk>  allele, to prevent strong eumelanin production in the iris. In these cases, your ultimate eye color depends on the alleles present at 2 other loci, EYCL1 and EYCL2. Green eyes<unk> e dominant over blue eyes, and these alleles<unk> e carried at the EYCL1 locus. Here, the pigment expressed is a yellow one, which combined with the bluish hue of a bit of eumelanin gives a green eye color. Persons whose eyes produce large amounts of both pigments - brown and yellow - will have amber eyes. This, incidentially, shows that just because an allele produces a dominant trait, it is not necessarily common: green and amber eyes, although the more dominant trait,<unk> e actually rarer than blue eyes. Simply put, the frequency of an allele is due to population genetics effects, whereas the dominant vs<unk> ness of a trait is due to how specific biochemical reactions<unk> e affected by the different alleles.
Note that it is phenes and phenotypes that<unk> e dominant and<unk> , not the alleles or genes. Note also that the term "dominant/recessive allele", while technically incorrect, is correct as regards the allele - locus/gene relationship in dominant-recessive inheritance. "Dominant/recessive gene" is utterly wrong in most of the (Mendelian) contexts it is usually encountered; for a gene to dominate over another in expression of a phenotype, epistatic or other forms of multi-gene expressions<unk> e required.
Autosomal recessive
X-linked dominant
Mosaic 
Extraneous variable
Nuclear isomer
Wild-type 
Short Stature
Down syndrome causes