Nearly all cells in the human body contain 46 chromosomes.  There are 2 copies of each of the 22 autosomal chromosomes, and 2 sex chromosomes.  Women have 2 copies of the X chromosome, the female sex chromosome, whereas men have one X chromosome and one Y chromosome, the male sex chromosome.  There are hundreds to thousands of genes on each chromosome.  Because there are 2 copies of the autosomal chromosomes, there are two copies of each gene present on them.  If the two gene copies are the same allelic form, the person is said to be homozygous for that gene.  If there are different alleles present, the person is said to be heterozygous for that gene.  For example, everyone has 2 copies of the PTC gene (the gene for tasting the bitter compound PTC).  There are 2 different alleles of this gene, the taster allele and the non-taster allele.  You might be homozygous for the PTC gene (have 2 of the same alleles) or you might be heterozygous for the PTC gene (have one of each kind of allele).