The three domain system adds three new categories called domains above kingdoms: Eukaryota, which contains all the organisms that have eukaryotic cells similar to ours such as plants, fungi, protists, and other animals; Bacteria, which refers to the very tiny single-celled prokaryotic organisms that are found almost everywhere; and Archaea, which were originally thought to be bacteria but are actually a different type of prokaryotic cell which are often found in extreme conditions like hot springs and salt lakes.