Protozoa are single-celled creatures with nuclei which show some characteristics, such as heterotrophy and motility, usually associated with animals. They are now generally included in the kingdom Protista, together with the plant-like algae. Some smaller groups of protozoa may also be treated as separate kingdoms.

The traditional division of protozoans is on the basis of movement, and while this is no longer believed to represent true relationships, it's still a useful way of organizing the many different groups:

Most protozoans are too small to be seen with the naked eye - most are around 0.01-0.05 mm, although forms up to 0.5 mm are still fairly common - but can easily be found under a microscope. Protozoa are ubiquitous throughout aqueous environments and the soil, and play an important role in their ecology. A few are also important parasites.