The Fundamental Question: What is it?
To many, cancer is a scourge of the old and the unlucky. We are constantly blasted with advertisements about "anti-cancer" foods and drugs that are supposed to help stave off the wretched giant-killer. While these are not necessarily false claims, they have created a perception of cancer as a foreign enemy. In truth, cancer is a traitor, an enemy from within!
If we may take a step backward, remember that your body is made up of cells. Each of these cells acts much like an individual organism: they must take in nutrients to survive and most of them can actually live apart from the body under laboratory conditions. They differ from "free-living" cells in that each one serves a purpose for the body as a whole. If a cell acts "out of turn" or becomes unnecessary, it undergoes a process called apoptosis that leads to its death. For a body cell, this is a normal part of the cycle of life.
Another limit on our cells is how many times they may divide. Division among single cells is akin to reproduction. A bacterium, for instance, will generally try to divide as often as it can in order to propagate itself in as many places as possible. In a highly regulated body, this is undesirable. Each cell has limits as to the number of times it may divide. Once it has used up all of its "turns," it stops growing and goes into a static phase.
As an example, the cells that make up the distinctive parts of the face replicate constantly for a certain period of time. Once they stop dividing, you're left with "what you've got." Exposure to UV rays, normal cell death, and other factors eventually begin to erode the number of skin cells. With fewer cells to hold the shape of your face, you get wrinkles.
What does all of this have to do with cancer? Well, remember when I said unregulated growth is bad news for a regulated body? If a cell finds a way to replicate endlessly and grow out of control, that IS cancer. For all intents and purposes, I could end the article here with that definition, but there are a few more important details to work out...