A tumor is an abnormal growth of cells that forms a lump or mass in the body. Your body is constantly making new cells to replace old ones, but sometimes this process goes wrong and cells multiply when they shouldn't, creating a tumor.

Not all tumors are dangerous. Benign tumors grow slowly, stay in one place, and usually aren't life-threatening. A common benign tumor is a wart-like growth on the skin. Malignant tumors, also called cancer, are more serious because they can grow quickly, damage nearby tissues, and spread to other parts of the body.

Doctors can often remove tumors through surgery, or treat them with special medicines or radiation. Scientists have made tremendous progress in treating tumors over the past 50 years, and many people who develop tumors recover completely after treatment.

When doctors examine patients, they check for unusual lumps or swellings that might indicate a tumor. Modern medical technology, like X-rays, CT scans, and MRIs, helps doctors find and identify tumors early, when they're often easiest to treat.