Answer:
Every single cell in the human body, or in almost any living species, for that matter, relies for survival on the process of gene replication, and cell division. Without this process, there would be no regeneration, and therefore, no life.
Because the process is very delicate, and it requires a perfect balance to ensure that everything from the DNA is replicated exactly as it was originally, so that errors are not made, and thus diseases can happen, there are also prevention and precaution systems in place inside living organisms that are meant to ensure that the replication of DNA is seamless and efficient.
Thus, when a cell initiates a process of division, the first step it will follow will be replication of its DNA, its genetic material, and enzymes will play a role in ensuring correct copying of information, the deletion of errors, and the release of new DNA strains that will work as they should in cell activities. When these safety processes fail, or when the DNA is exposed to external injury, like when it is exposed to U.V light, or to chemicals that can change and affect its sequence, then copying and replication errors are committed and in many cases, the systems of precaution that were in place, cease to function.
Such is the case of cancer and tumor formation. Tumors, are just body cells whose precaution systems did not work, and whose DNA has been mutated to produced faulty proteins that will not work as they should. These cells, normally, should go into apoptosis, and die. But because of mutations, the system does not work. Two characteristics of these tumor cells are that they are not detected by the immune system because they are basically the body´s own, and two, they grow and divide without stop because the corrective systems no longer work on them.
Metastatis, then, is when a group of tumorous cells grow without control on a specific place of the body and rupture the tissue where they are located simply because they no longer fit, and need new spaces to continue developing. Usually they rupture the tissue and infiltrate the blood, or lymphatic vessels and settle themselves in new tissue, which they start to invade.
This is the process of mutations, tumor formation, and metastasis.