This video is from BioDigital's website.
What is Cancer?
In an adult human's body, there are approximately 200 to 250 trillion cells. Except for red blood cells, all of these cells have the capability of reproducing, or making copies of, themselves. Each day, many cells in your body die, and are replaced. Skin cells, for example, are replenished rapidly; in an adult, the body produces the exact same amount of skin cells that are lost daily. Cancer occurs when one or cells experience a mutation that causes the cell or cells to divide (reproduce) uncontrollably. The result is a tumor. There are two types of tumors: benign and malignant. Benign tumors are growths of cells that lack a blood supply; in other words, they don't receive many nutrients or oxygen. As a result, benign tumors cannot grow beyond a certain size. Malignant tumors are growths that receive plenty of nutrients and oxygen; malignant tumors force the body to create a blood vessel or blood vessels that run directly to the tumor. Because they possess a large store of resources, malignant tumors have the potential to grow extremely fast, and therefore are cancerous. Malignant tumors can also spread quickly to different parts of the body using their personal blood vessel(s). In fact, this is usually the main way a cancer kills a person. Think of a bookshelf: only so many books can fit on a bookshelf. In cancerous tumors, there are too many cells in a given area, so the large growths of cells crowd out other cells in the body. Benign tumors, on the other hand, are not cancerous, but can sometimes become cancerous through various malfunctions.
Below is a video from the Khan Academy website that explains cancer in a more detailed manner.