Cancer can either be a genetic - meaning you get it from your parents - or an exposure to environmental things like smoking or the sun called carcinogens.
Everyone has a different combination of genetic changes that lead to cancer. In a tumor genetic changes are still even taking place. These changes can be mutations in DNA. There are two types of tumors benign and malignant. Benign tumors are not cancerous but malignant tumor are cancerous growth.
Cancerous cells spreading
Changes to these three genes lead to cancer: proto-oncogenes, tumor suppressor genes and DNA repair genes.
DNA repair genes
Proto-oncogenes are involved in normal cell growth and division. When these genes are altered or are more active than normal, they may become cancer-causing genes (or oncogenes), allowing cells to grow and survive when they should not.
Tumor suppressor genes control cell growth and cell division. Cells with alterations on thees tumor suppressor genes could divide uncontrollably.
DNA repair cells fix damaged DNA. Cells with mutations in these genes tend to develop additional mutations in other genes. Together, these mutations may cause the cells to become cancerous.