Microscopic Findings
The four Plasmodium species that cause human malaria can be distinguished most of the time (but not always) based on the morphology of their blood stages.  The Table describes their most salient morphologic characteristics, in each of 4 morphologically distinguishable stages:
Ring: early developmental stage of the asexual erythrocytic parasite; the term is derived from the morphologic appearance of this stage, which includes chromatin (red), cytoplasm (blue), often arranged in a ring shape around a central vacuole; biologically, the ring is a young trophozoite.
Trophozoite: next developmental stage of the asexual erythrocytic parasite; it has lost its "ring" appearance, and has begun to accumulate pigment (colored yellow to black).
Schizont: late developmental stage of the asexual erythrocytic parasite; it has begun its division into merozoites, and thus is characterized by the presence of multiple contiguous chromatin dots (to be distinguished from multiple chromatin dots from multiple infections, which tend not to be contiguous).
Gametocyte: sexual erythrocytic stage.

Other Parasite Stages Seen Rarely in Blood: There are some other malaria stages that can be seen, but are rare and/or do not assist in diagnosis.  They are the merozoites, which are released when the schizont ruptures, and which will reinvade new erythrocytes to continue the asexual erythrocytic cycle; and on exceptional occasions, when blood samples are left at room temperature, the gametocytes can exflagellate and the blood smears can have exflagellating gametocytes, microgametes, or even ookinetes (the product of the fusion of a male and female gametes, usually found only in the mosquito).