Multiple myeloma, also known as plasma cell myeloma, is a cancer of plasma cells, a type of white blood cell normally responsible for producing antibodies. Often, no symptoms are noticed initially. When advanced, bone pain, bleeding, frequent infections, and anemia may occur. Complications may include amyloidosis. The cause is unknown."
Source : Wikipedia
Although the cause (or causes) are not yet determined, several 'risk factors' (i.e. conditions which might increase the risk of contracting the disease) are known - listed here.