Leukemia is a type of cancer that originates in the bone marrow, usually beginning in the cells that produce blood cells. It involves the production of high numbers of abnormal blood cells, which are not fully developed and called blasts or leukemia cells. Leukemia is broadly classified into four main types based on the speed of progression (acute or chronic) and the type of blood cell affected (lymphocytic or myeloid):

• Acute lymphoblastic leukemia (ALL)
• Acute myeloid leukemia (AML)
• Chronic lymphocytic leukemia (CLL)
• Chronic myeloid leukemia (CML)

Acute leukemias grow rapidly and involve the rapid increase of immature blood cells, requiring immediate treatment. Chronic leukemias develop more slowly and involve an excessive buildup of more mature but still abnormal white blood cells. Lymphocytic leukemias affect lymphoid cells (which normally develop into lymphocytes, a type of white blood cell), while myeloid leukemias arise from cells that normally form red blood cells, some white blood cells, and platelets. In sum, leukemia is a blood cancer affecting blood-forming tissues, primarily distinguished by the type of blood cell involved and the disease's progression speed.