Several features of the blood are measured in the complete blood count.

The Red blood cell evaluates

Red Blood Cell Count: Red blood cells (RBCs), also called erythrocytes, are cells that carry oxygen from your lungs to the rest of your body. The RBC counts the number of red blood cells that are present in your blood sample. If your RBC count is too low, you may have anemia or another condition.
Hemoglobin: It is the oxygen-carrying protein in red blood cells that gives the cells their representative red color.
Hematocrit:  It measures the volume percentage of RBC (Red Blood Cells) in the blood. A hematocrit value of 30% means that there are 30 milliliters of RBC in 100 milliliters of whole blood. 
The CBC blood test also provides important information on various red blood indices including

Mean corpuscular volume (MCV) is a measurement of the average size of red blood cells.
Mean corpuscular hemoglobin (MCH) is a calculated measurement of the average amount of hemoglobin inside red blood cells.
Mean corpuscular hemoglobin concentration (MCHC) is a calculated measurement of the average concentration of hemoglobin inside red blood cells.
Red cell distribution width (RDW) is a measurement of the variation in the size of red blood cells.
 

White blood cell tests

White blood cells (WBC) count:  It defines the total number of white blood cells in your blood sample. WBC helps your body fight against infection. A CBC measures the number and five types of white blood cells present (neutrophils, lymphocytes, monocytes, eosinophils, and basophils). 

Platelet tests

Platelets(thrombocytes) assist in blood clotting. The platelet count is a test that measures the number of platelets in your blood sample.
Mean platelet volume (MPV) is a measurement of the average size of platelets.

The CBC blood test is a great indicator of overall health and detects a variety of diseases and conditions, such as infections, anemia, and leukemia. Any increase or decrease in cell counts can indicate an underlying medical problem. It can give you and your health provider a heads up that further investigation is required. A CBC is also done to keep track of an existing medical condition or treatment.