The main thing that distinguishes cancer cells from normal cells is uncontrolled growth. Healthy cells will reproduce only when the body needs them to do so. Cancer cells have escaped from the body's normal control systems and continue to reproduce over and over, populating an area of the body with too many cells, which are often of the wrong type for the location they are in. Because of their rapid rate of reproduction, many cancer cells do not mature and differentiate properly, so they are essentially useless to the body.
In normal cells, cells reproduction is often density dependent. This means that crowding suppresses cell division. Cancer cells are rarely affected by density, retaining their ability to divide even under crowded conditions.
Cancer cells are able to initiate a process called angiogenesis, which causes the body to grow extra blood vessels into areas where the cancer cells are proliferating, which gives the cancer better access to oxygen and nutrients and speeds tumor growth. Many cancer cells are also capable of metastasis, which is the ability of these cells to spread into and colonize areas far from their site of origin.