An amoeba is a generally microscopic type of protozoa, a single-celled organism. There are a variety of types, and their feeding habits vary. For instance, there are parasitic amoeba such as the "brain-eating amoeba" (Naegleria fowleri), as well as those that eat algae, plant cells and other protozoa.
According to the Enchanted Learning website, they live in a variety of environments: fresh water, especially puddles and ponds; salt water, wet soil and in animals.