Epistasis is a phenomenon by which the phenotype of one gene gets modified by one or several genes. The gene/s which causes the modification is known as modifier genes.
The gene whose phenotype is expressed is called epistatic, while the phenotype altered or suppressed is called hypostatic.
Classic example is the coat color in labrador retrievers which is controlled by two genes, one for color, and one which allows color to be expressed. The gene for coat color is generally called "B" and the dominant form is black, the recessive form (b) is chocolate. The "expression gene" when dominant ("E") allows the coat to be these colors, but the recessive form (e) results in the darker colors not being expressed, resulting in a yellow lab.
Dominance on the other hand is a relationship between two alleles of a single gene, in which one allele masks the effect of the other in influencing some trait. In the simplest case, if a gene exists in two allelic forms (A & a), three combinations of alleles (genotypes) are possible: AA, Aa, and aa. If Aa individuals (heterozygotes) show the same form of the trait (phenotype) as AA individuals (homozygotes), and aa homozygotes show a different phenotype, allele A is said to dominate or be dominant to allele a, and a is said to be recessive to A.
By one convention, dominant alleles are written in uppercase letters, and recessive alleles in lowercase letters.
Example is that classis Mendelian experiments where height of the pea plant denoted by two alleles H(long ) and h(short). H being dominant over h.