Genes come in pairs, called alleles, and each pair is located in a specific position (or locus) on a chromosome. If the two alleles at a locus are identical to each other, they are homozygous; if they are different from one another, they are heterozygous.
Like all words with the prefix hetero, heterozygous has to do with things that are different — specifically genes. When alleles are heterozygous they are unalike in some way. Alleles determine the genotype, or genetic structure, of a specific gene. For example, you may have received the allele for brown eyes from your mom and the allele for blue eyes from your dad; that means that you are heterozygous for eye color. Whether an organism's genotype is heterozygous or homozygous helps to determine its appearance (which is called its phenotype).