Leukemia is a cancer of the bone marrow, which directly effects the blood.  To understand this one must know the function of the bone marrow in relation to blood:

Bone Marrow does many things for the body, its three main functions are producing:

Red Blood Cells:  Also known as erythrocytes, they carry oxygen throughout the body
White Blood Cells:  Also known as leukocytes, they defend the body from viruses and other infections
Platelets:  Also known as thrombocytes, they stick to the inner sides of vessels plugging up minor leaks, they also release a substance for clotting blood


There are four main catagories of Leukemia.

Chronic:   With Chronic Leukemia, symptoms may not occur for a period of time.  The cells are able to function normally without trouble for many years, however, over time symptoms occur when the number of leukemia cells increase within the blood.

Acute:   With Acute Leukemia the blood cells are very abnormal, and the amount of them increase rapidly.  Because of the growth the cancer worsens quickly.

Lymphoctic:  When leukemia effects the white blood cells called lymphocytes, which form from the lymphatic tissue.

Myelogenous:  Occurs when the cancer afects the myelocytes, a specific type of white blood cell.

Leukemia can occur within these four types.

Chronic Lymphocytic Leukemia:  This type accounts for about 7,000 of new leukemia every year.  It rarely effects children, often occuring in middle aged people.

Chronic Myeloid Leukemia:  A type that occurs mainly in adults, it effects 4,400 cases annually.

Acute Lymphocytic Leukemia: With about 3,800 cases every year, Its most common in children, however it can also effect adults.

Acute Meyeloid Leukemia: With 10,600 cases every year, it is the most common form.  It occurs equally in adults and children.