A benign tumor sits there and grows slowly. If you remove it, it doesn't return.
Cancer is a malignant tumor, which invades tissues aggressively.
It may only invade locally, or it may metastasize and spread to areas far away, like the lungs and liver. The most malignant types grow rapidly and have already spread or metastasized to other organs before the original tumor is detected. So a malignant tumor of the skin that is only locally invasive can have a better outlook than a benign tumor of the brain. The mild types have a more hopeful outlook than the severe types. There is nothing that will always cause cancer in all animals.