How exactly does a tumor work? how does it form and what does it do that is bad for the body?

Altered normal cell: normal cells can be affected by viruses or carcinogens to develop mutations in certain genes termed "oncogenes" Certain of these genes like p53 allow cells that have a DNA mutation to slow their rate of division to allow corrections. With mutated genes the corrections not made and mutation is passed to divided cells. There are enzymes like telomerase to allow the mutated cell to have immortality.