Tumor - abnormal growth. Malignant - harmful or dangerous. The word malignant is most often used to describe cancer.
Cancer is the direct invasion of bad cells into normal good cells.
These bad cells grow and divide until they eventually choke out the good cells much like weeds invading your garden. The weeds keep growing and take up space and nutrients which your garden needs to survive. At some point in time, when there is no more food or space for your beautiful flowers they will die unless you pull the weeds or apply weed killer.