Cancer is so awful, but I really don't know how it works or why its so hard to cure.
Cancer is kind of an umbrella term for a group of diseases that involve abnormal cell growth. The cells go out of control and invade the other parts of the body. 
Ohh so is it just hard to near impossible to stop the intrusive cell growth?
If you remove one tumor, there's the chance another one will grow somewhere else. Benign tumors, in contrast, don't spread. They're easily removed and treated.