The difference between a benign and malignant tumor is that a benign tumor doesn't metastasize, or spread to distant sites, according to MedlinePlus. Benign tumors can often be removed with surgery. Sometimes, they do not grow back, though tumors such as fibroids tend to recur, explains WebMD.
One reason benign tumors don't spread is that the cells tend to stick together, according to About.com. The cells of malignant tumors lack this characteristic, so they break off and spread. When looked at under a microscope, malignant and benign cells also look different.
Just because some tumors are benign does not mean they are harmless. A benign tumor in the brain can be nearly as dangerous as a malignant tumor, claims the American Brain Tumor Association. Indeed, an otherwise benign tumor that's found in a part of the brain that controls vital functions is considered malignant, even though it doesn't metastasize. Even malignant brain tumors rarely spread to distant sites in the body, but they grow quickly and invade the brain tissue.
Benign tumors can also grow very large and interfere with the functioning of other organs, states Johns Hopkins Medicine. There are cases of benign ovarian tumors that weighed over 100 pounds, notes About.com.