Cancer is an aberration of normal development and cell physiology. In other words Cancer is a disease by our own cells developing or "going" wrong. Cancer cells have a lot of the characteristics of normal cells. The cancer cell exhibits many of the behaviors found in normal cells during development. However, these similar properties are separated from normal controls and combined in unique and troublesome ways. Differentiation is the "grade" of a cancer. The more normal a cancer cell looks like the lower its grade. A low-grade cancer is less likely to be aggressive. Looking at the cell under a microscope and seeing how developed or mature it is determine this. The more a cell looks like a normal one the more it will behave like one. .
Normal Cells have a number of important characteristics. They can reproduce themselves exactly. They can stop reproducing at the right time. They can stick together in the right place. They will self destruct if damaged. They become specialized or mature. Cancer cells differ from normal cells in that they do not stop reproducing. The Cancer cell keeps reproducing itself going on and on doubling. They may be more resistant than normal cells in that they self-destruct slower than they reproduce. Eventually a tumor is made up of billions of the original cancerous cell. They don't" respond or "obey" neighboring cells. Something in the cancer cell overrides the normal signaling system. This may be because the genes that tell the cell to stop reproducing have been damaged or lost. So the cancer cell keeps on reproducing regardless of the damage it is doing to the part of the .
body that it is growing. They don't stick together or become specialized or mature. They don't die if moved to another part of the body. Cancer cells loose the molecules on their surface that keep normal cells in the right place. This makes them become detached and stray. This is why cancer spreads to other parts of the body.