After an erythrocyte is penetrated by a merozoite, the parasite loses its penetration organelles and begins feeding and growing as a trophozoite. In the young trophozoite, the large central vacuole surrounded by the thin band of cytoplasm, and single chromatin dot on the periphery, give it the appearance of a "signet ring" or "pinky ring", hence the common name "ring stage."
This photo shows a ring stage trophozoite of P. falciparum within an erythrocyte. This species is never associated with any stippling or pigmentation of the host cell.