Leukemias are grouped by how quickly the disease develops as well as by the type of white blood cell that is affected. The two main types of leukaemia are acute and chronic. Acute leukemia is a rapidly progressing disease that affects immature blood cells (blasts) which are not yet fully developed. These blasts cannot carry out their normal functions, increases quickly and the disease gets worse quickly. Acute leukaemia tends to affect younger people. In chronic leukemia, some blast cells are present, but they are more mature and can carry out some of their normal functions. The number of blasts increases less rapidly than in acute leukemia and therefore the disease gets worse slowly.

Leukemia can arise in either of the two main types of white blood cells: lymphoid cells or myeloid cells. Leukemia that affects lymphoid cells is called lymphocytic leukemia and myeloid cells is called myeloid or myelogenous leukemia.

The most common types of leukemia are :

Acute Lymphocytic Leukemia (ALL) - The most common type in young children under 19 years. Also affects adults of 65years and older.
Acute Myelogenous Leukemia (AML) - Found in both adults and children. AML is also called acute myelogenous leukemia, acute myeloblastic leukemia, acute granulocytic leukemia, and acute nonlymphocytic leukemia. There are different subtypes of AML based on how mature (developed) the cancer cells are at the time of diagnosis and how different they are from normal cells.
Chronic Lymphocytic Leukemia (CLL) - Occurs in adults over 55 years. It almost never affects children.
Chronic Myelogenous Leukemia (CML) - Affect mainly in adults.

Human T-cell leukemia and Hairy cell leukemia are less common type of chronic lymphocytic leukemia. Hairy cell leukemia is called so because when viewed under a microscope, these cells appear to be covered with tiny hair-like projections.