The good news is that most human skin cancers are benign. The bad news is that most cat skin cancers are malignant, and they are also the second most common cancer found in cats. (Lymphoma is the first.) They are more likely to occur in older cats, and the cats most at risk to develop it are white cats, especially those with blue eyes.
The danger of skin cancers is their ability to spread to other organs, usually the lymph nodes first, then the lungs. Vets try whenever possible to remove the cancers surgically. Obviously, as with human skin cancers, the earlier the cancer is found, the more likely that treatment will be successful.