A diploid number is a term in biology that refers to the number of chromosomes in a given cell. For example, a human cell has a diploid number of 46. It has two sets of 23 chromosomes.
Chromosomes are the protein structures that carry DNA. They are long and thin and stored in the nucleus of a cell. In humans, chromosomes come in pairs that are joined at a centromere, forming a squashed X shape. Each of these chromosomes in a pair is called a chromatid. The only cells that do not have pairs of chromosomes are gametes, or reproductive cells: eggs and sperm. These cells have singular chromosomes, and when they join with a gamete of the opposite kind, their chromosomes link up to form chromatid pairs.
Chromosomes are capped with telomeres, which are essential to the structure of the chromosome. Telomeres decay with each cell replication. Once the telomere is completely decayed, the cell can no longer replicate and dies. Cancer cells contain enzymes which prevent telomere decay, and this is one reason they grow so quickly.