A cell is diploid if it contains exactly two copies of each chromosome.
Ploidy is used to describe how many copies of each chromosome an organism's cells have; many organisms have several types of cells with different ploidy. The basic set of each chromosome is referred to as the monoploid number; in diploid cells, two copies of each chromosome are found; there are also triploid and tetraploid cells. Another common type of ploidy, haploid, refers to cells with only one copy of each chromosome. Most sexual reproduction takes place when a diploid cell is formed from two haploid cells. In organisms that do not reproduce sexually, but standardly contain only one copy of each chromosome, the term monoploid is often used in the place of haploid.
Almost every animal has a predominant number of diploid cells. In organisms that reproduce sexually, the two copies of each chromosome usually come independently, one from the male and one from the female. This gives the organism that critical advantage that emerges from sexual reproduction: the opportunity to mix up genes.
A few species have haplodiploid cells; this is where one sex (typically the male) has haploid cells and one sex has diploid cells. In these species, the males develop from unfertilized eggs, while every fertilized egg develops into a female, with two complete sets of chromosomes. This is commonly found in insects that have a queen, such as in ants, bees, and wasps.
Researchers can actually use diploid cells to create haploid cells by manipulating the diploid cell to extract only one set of chromosomes.
Web Resources On Diploid Cells
The Haploid-diploid Cycle of Sexual Reproduction
Alternation of Generations
Book Resources On Diploid Cells
Transformation of Human Diploid Fibroblasts: Molecular and Genetic Mechanisms by George E. Milo
Molecular Biology of the Cell by Bruce Alberts, et al