1. Bacteria: These are single-celled prokaryotic organisms, meaning they lack a nucleus and other membrane-bound organelles. They are incredibly diverse and found in almost every environment on Earth.

2. Archaea: Like bacteria, archaea are single-celled prokaryotes. However, they have distinct molecular characteristics that set them apart from bacteria, such as unique cell walls and ribosomal RNA. They are often found in extreme environments, like hot springs or salty lakes.

3. Eukarya: These are organisms whose cells have a nucleus and other membrane-bound organelles. They include all multicellular organisms (plants, animals, fungi) as well as many single-celled organisms (like protists).

It's important to note that these are the three main domains of life, not just cell types. Each domain contains a wide variety of organisms with different cellular structures and functions.