Chromosomes are structures in the cell nucleus that carry the DNA. They divide when the cell divides - this process is called mitosis. They can be seen through a light microscope at certain stages in the cell cycle.
Individuals of different species characteristically have different numbers of chromosomes - each individual human has 46, for example, whereas a fruitfly Drosophila melanogaster has eight, and other species have other numbers.
The image opposite is of a human genome arranged along strings of chromosomes.