Protozoa (greek, protos-first, zoon-animal) are single-celled (unicellular) eukaryotes generally grouped under Kingdom Protista. Protozoans were among the earliest life forms on earth and have a simple body plan. Most of the protozoans are very small, and can only be seen through a microscope. They live in soil and aquatic environments such as lakes (fresh water) and oceans.
Protozoans exist as individuals, but some are colonial.
All the metabolic activities of an organism like respiration, digestion, excretion and reproduction are carried out by these single celled organisms.