A cell's genetic information, in the form of DNA, is stored in the nucleus. The space inside the nucleus is limited and has to contain billions of nucleotides that compose the cell's DNA. Therefore, the DNA has to be highly organized. There are several levels to the DNA packaging.
At the finest level, the nucleotides are organized in the form of linear strands of double helices. As you zoom out, the DNA strand is wrapped around histones, a form of DNA binding proteins. Each unit of DNA wrapped around a histone molecule is called a nucleosome. The nucleosomes are linked together by the long strand of DNA.
To further condense the DNA material, nucleosomes are compacted together to form chromatin fibers. The chromatin fibers then fold together into large looped domain. During the mitotic cycle, the looped domains are organized into distinct structures called the chromosomes.
Chromosomes are also used as a way of referring to the genetic basis of an organism as either diploid or haploid. Many eukaryotic cells have two sets of the chromosomes and are called diploid. Other cells that only contain one set of the chromosomes are called haploid.
The chromosome also plays an important role in cell death-related aging phenomena. At the tips of chromosomes are segments called telomeres. As a cell's DNA is damaged, the telomeres are shortened. Once the telomeres have been reduced to a level, the cell decides that it can no longer repair itself and initiates apoptosis, the cellular death process. Today, much research efforts have been devoted to elucidating the specific mechanisms by which telomeres cause cell death.