A haploid cell bears one set of chromosomes, a diploid cell bears two sets, and a polyploid cell bears many sets.
The ploidy of cells can vary within an organism. In humans, most cells are diploid (containing one set of chromosomes from each parent), though sex cells (sperm and oocytes) are haploid.
Aneuploidy is when a cell contains an abnormal or non-integer ploidy number. This may lead to problems in cell development. Most forms of aneuploidy in humans are lethal, but trisomy (three copies) of the sex chromosome (the cause of Klinefelter's syndrome and others) and of chromosome 21 (the cause of Down syndrome) are relatively common.
Many forms of cancer have incorrect ploidy numbers, due to the accumulation of mutations which increase chromosome missegregation.