As you've learned, classification is always changing. Currently there are 6 Kingdoms in our classification system. 

We began with only two, Animalia and Plantae.  Currently we also consider Protista (single celled organisms), Fungi, Bacteria, and Archaea Kingdoms in our classification system. 

Domains are above kingdoms in the hierarchy, and are based on molecular evidence. The three domains are: Bacteria, Archaea, and Eukarya. 

• Bacteria - prokaryotes in the kingdom Bacteria
• Archaea - prokaryotes in the kingdom Archaea. These organisms are chemically different from bacteria and live in extreme environments
• Eukarya - this domain includes all eukaryotes! Remember that eukaryotes have a membrane-bound nucleus and organelles.