The body is made of many building stones, cells included. Cells multiply a limited number of times after which they die. The old cells are then replaced by new ones.
The more a cell multiplies, the higher the risk becomes that these multiplications are accompanied by mutations/mistakes. These mistakes or mutations can be the result of a coincidence, but also of exposure to environmental factor such as UV radiation or smoke particles.
Normally, these mistakes are recognized by the control mechanisms of the cell. The mistake can then be fixed or the cell dies. Sometimes the cell can escape these control mechanisms and start to multiply without limits. At that moment, the cell has become a cancerous cell.
When cancer cells multiply in a uncontrolled way, a tumor forms. The more cells multiply, the larger the tumor gets. The faster the cells multiply, the more malignant/high-grade the tumor becomes.
When the tumor reaches a size of 1-2 millimeter, the cancer cells need blood vessels to continue growing. They are capable of influencing the surrounding tissue in such a way that it will allow new blood vessels to be formed in the direction of the newly formed clump of cancerous cells.