Metatstatic cancer is a specific type of malignant cancer, where the aberrant cells metastasize to different organs in the body. While all metastatic cancers are malignant, if a malignant tumor doesn't move in the body it isn't considered metastatic.
In most every case, this is the action that kills the host. Once the cancer starts moving, it's a toss up as to which vital organ the tumor either strangles or damages past the point of survival. In many cases it is the brain or the lungs.
The major problem with this paticular type of disease is it must be treated in the same manner that the host organ was. For example if a patient has breast cancer in the lungs, it must be treated with breast cancer drugs or therapies (see tamoxifen) and not with lung cancer techniques.