Hematology is the study of blood. A hematologist is an IM physician who has completed a 2 yr fellowship in that specialty. Most hematology fellowships are combined with oncology fellowships because there is a lot of overlap (3 yrs for the two of them), and quite frankly, I don't think that there is a large demand for hematologists alone in non-academic medical centers (I could be mistaken, I just don't think that they get consulted that often in community hospitals and I don't think that there are enough heme cancer patients in most areas to support their practices). Anyways, they mainly deal with heme cancer patients (leukemias, lymphomas), but they can also see patients with a variety of other hematologic disorders (hemophilia, thrombocytopenia or anemia of unknown etiology, etc). Here are some helpful websites:
http://www.hematology.org/about/defining.cfm
http://www.acponline.org/srf/subspecialty_brochure.htm#hemo
http://www.hematology.org/about/defining.cfm
http://www.acponline.org/srf/subspecialty_brochure.htm#hemo