The chromosomes are paired, so that if for example a gene for eye colour is on one chromosome in a pair then there is also a gene for eye colour on the same location, same "locus", on the other chromosome. It is not necessary the same predisposition for a certain eye color on both chromosomes, but it might be. Genes with similar functions are called homologous.
When a cell divides itself a doubling of the number of chromosomes takes place in that each and every of the chromosomes splits itself in half along the middle into a group of two. The pair is kept together in a certain place called the centromere. Since each cell has two copies of each chromosome to start with, this means that every cell, just before division, has 4 copies of each chromosome. The two original chromosomes of the pair are similar, but the two copies of each chromosome that are held together at the centromere, are identical! After this doubling, the centromeres line up in the center of the cell, where the centromeres split: the two chromosomes of the identical pair, now finally released from each other, moves to opposite sides of the cell. Since all chromosomes do this, the chromosomes are divided into two identical groups, each group moving to opposite sides of the cell to form the nucleus of the daughter cell. The cell wall is constructed between the two daughter cells and the process can start all over again. This type of cell division is called mitosis. Thus, during the mitosis the mother cell is divides into two identical daughter cells, which both have exactly the same set of chromosome as the mother cell.
There is also another type of cell division: reduction division or meiosis. During meiosis the numbers of chromosomes are reduced so that the daughter cells only contain half as many chromosomes as the mother cell, one chromosome from each pair. These cells with a single set of chromosomes are said to be haploid (Greek: haploos=single). Cells which have sets of double chromosomes, that is all chromosomes in pairs, are called diploid (Greek: diploos=double). In higher organisms only gametes, reproductive cells like eggs and sperms, are haploid. If the gametes were not haploid the number of chromosomes would double itself in each generation.
In several organisms, including mammals, there are two of the male chromosomes that do not form a homologous pair. They are called the x- and y-chromosomes. During the meiosis, two types of sperm are always formed, one with an x-chromosome and one with a y-chromosome. The female cells contain two x-chromosomes and each egg contains one of these. When an egg and sperms melt together during conception the chromosomes are mixed together and the diploid order is restored. One chromosome of each pair of chromosomes is hence from the mother and the other one from the father. The fertilized egg starts growing by mitosis. The sex of the new organism depends on whether the egg has been fertilized by a sperm with an x-chromosome, which gives a female, or a y-chromosome, which gives a male. Among certain species, birds for example, it is the other way around in that it is the egg cell that has two different gender deciding chromosomes.