Blood consists of plasma (fluid) and formed_elements ("cells"). Blood "cells" are called formed elements because only one of the three types of "cells" actually has complete cellular structure at maturity.
Platelets are cell fragments that lack a nucleus; therefore they are not "true cells" because they lack complete cellular structure at maturity. Platelets assist with the process of coagulation.
Leukocytes retain their nuclei and organelles at maturity and are therefore "true" cells. Leukocytes are defensive cells and are also referred to as WBC's or white_blood_cells.
WBC's can be classified based upon their variations in appearance. Two factors are involved: (1) the shape of the nucleus and (2) the presence or absence granules (red, blue or both) within the cytoplasm of the cell. Granules are actually cell organelles that react to chemicals used in the staining process.
There are two classes of WBC's: (1) agranulocytes, which lack distinctive granules in their cytoplasm and (2) granulocytes, which contain red or blue (or both) staining granules in their cytoplasm.
Monocytes are agranulocytes and have a round nucleus that looks like a pie with a slice missing. Monocytes specialize in the process of phagocytosis. When they move into tissue spaces they are termed macrophages.
(1) When viewing a stained prepared blood slide, RBC's will look small, anucleate and biconcave; platelets will appear as tiny fragments; and WBC's will vary in appearance, but will all have a well-developed nucleus.