Cancer is a group of diseases that begins in our genes. The genes in a healthy body work together, regulating cell division to ensure that each new cell is an exact replica of the parent cell. When cells are damaged and die, they are repaired and replaced in this way. Due to causes largely remain unknown, however, mutations in genes lead to cellular chaos: cells grow and divide unchecked, and cancer is born. As the abnormal mass of cells grows, blood vessels form to provide the tumor with nutrients. This is called angiogenesis. As the tumor continues to grow, it invades healthy tissue and can spread, or metastasize.
A risk factor is anything that increases a person's chance of getting a disease. Different cancers have different risk factors. However, having one risk factor, or even several, does not necessarily mean that a person will get cancer. Genetic, hormonal and immune factors and environmental factors (either working alone or in combination) affect the initiation and promotion of cancer.
National Cancer Institute: What You Need to Know About Cancer