DNA is an extremely long molecule. The DNA in a single human cell can stretch out to about 6 feet. Special packaging is needed so the DNA does not turn into a tangled mess!
DNA is divided into long pieces that are wrapped around proteins. These structures are called chromotids and are too thin to be seen with a standard microscope. The DNA is wrapped into even tighter coils when the cell is going to divide. At this point they can easily be seen in a microscope and are called chromosomes.
Each type of organism has a specific number of chromosomes. For instance people have 46 chromosomes and elephants have have 56 chromosomes. Chromosomes are not all the same size, so the number of chromosomes does not tell you how much DNA there is.
Animation of how DNA is coiled into chromosomes we can see through the microscope. (Howard Hughes Medical Institute)