Final answer:
Tumors spread to other parts of the body through a process called metastasis, where cancer cells from a primary tumor establish new tumors in distant tissues, characterizing malignant tumors.
Step-by-step explanation:
Tumors invade other organs and tissues in a process called metastasis. This phenomenon involves cancer cells shedding from the primary tumor, entering the bloodstream, and traveling to different parts of the body. There, they can establish new tumors, and this ability to metastasize is a defining characteristic of malignant tumors. Metastases compromised of migrated cancer cells often have the worst prognosis and are responsible for most cancer-related deaths.