Complete blood count (CBC) is a common series of blood tests that can be done on a fairly small sample of blood taken from a vein, usually at the inside of the elbow.
The tests most often performed include a red blood cell count, a white blood cell count, a differential white ell count, a platelet count, and a blood smear, which reports any abnormalities in the red blood cells.
The actual counting of cells is usually done by an electronic counter, in which a computer recognizes the patterns of each cell type. In some laboratories, parts of the CBC may still be done manually. 
A complete blood count is a starting point in the exploration of possible diseases or conditions.
A physician may request additional blood tests at the same time, or the CBC results may suggest areas for further investigation.