I fail to see why this was such a horrible question. It's not like people really actually explain cancer in simple terms when you see it on TV or hear about it. It's just "ohh, he or she has cancer...ooohhh"
My wife had thyroid cancer, in 6 months one cancerous tumor grew 20x as large, which is a very fast growth rate, indicating malignant cancer. She had her whole thyroid removed and was tested in her lymph nodes for spreading.
Which is the biggest danger of cancer. Fast growing cells that spread. Without removing them they can spread and grow very large, impeding organ functioning as they suck blood, nutrients, oxygen, and kill other cells around it. Slowly it will destroy any tissue if unchecked as it grows.
Brain cancer is devistating because removing tumors means doing brain surgery, a risky proposition at best. Without removing the tumor it'd eventually grow very large and destroy the brain. Lungs are the same way.
If it spreads too quickly, it can get everywhere and destroy the whole system. Operating and removing everything is very risky, as the blood loss, organ damage, and general shock to the system could be overwhelming, causing the whole body to shut down and die.
The cure for cancer, surgery and then radiation and chemotherapy is often just as devastating than cancer. Radiation and chemo is like taking a massive bomb to the body in an attempt to kill off a few cancerous cells. It doesn't just attack cancer, but everything else too. That is why it's so hard to treat. Radiation and chemo leave the person weak, exposing them to outside ailments and other cancers.
New treatment avenues include "smart bombs" which attack just the cancerous cells. Experiments in choking off oxygen to cancerous cells have proven somewhat successful, localizing treatment, rather than the nuke.
What they did to my wife was give her radiated iodine. The thyroid sucks up most of the iodine in your body, if they radiate it the iodine is absorbed and the radiation kills the thyroid cells that absorbed it. Kind of like a heat-seeking missile to put it into the most simple terms.
Nevermind people on here, most lack any type of empathy.