Malignant cells tend to regrow very rapidly, rather multiply, thus resulting in cancerous tumor. Whereas, benign tumor may be large, but do not multiply or spread further. Whether the tumor is malignant or benign can be diagnosed by biopsy, a kind of clinical examination. Benign tumors can be removed by surgical procedure, whereas malignant tumor, even if removed leave at least some cells, which again multiply to form cancerous cells. More details, you can have from your doctor.