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. 
Damn that's insane. So how does chemo work? Does the radiation slow or stop the growth of these cells?
Chemotherapy may be given with the intent of curing the cancer, usually with lots of other drugs, but it also can just be to prolong life or make symptoms less severe. 
That's so sad that we haven't found a cure for such a terrible disease, especially with how the best course of action is very harmful to the body anyway.
estimates, two in five people will have cancer at some point in their lifetime.
Complete removal of the cancer without damage to the rest of the body (that is, achieving cure with near-zero adverse effects) is the ideal goal of treatment and is often the goal in practice. Sometimes this can be accomplished by surgery, but the propensity of cancers to invade adjacent tissue or to spread to distant sites by microscopic metastasis often limits its effectiveness; and chemotherapy and radiotherapy can have a negative effect on normal cells
age and as lifestyle changes occur in the developing world. The financial costs of cancer were estimated at $1.16 trillion USD per year .
Definitions.
Cancers are a large family of diseases that involve abnormal cell growth with the potential to invade or spread to other parts of the body. They form a subset of neoplasms. A neoplasm or tumor is a group of cells that have undergone unregulated growth and will often form a mass or lump, but may be distributed diffusely.
All tumor cells show the six hallmarks