1. Archaea: Single-celled prokaryotes (lacking a nucleus) that often live in extreme environments.

2. Bacteria: Single-celled prokaryotes that are diverse and found everywhere.

3. Protista: Mostly single-celled eukaryotes (with a nucleus), but also include some multicellular algae.

4. Fungi: Heterotrophic eukaryotes that absorb nutrients from their surroundings, often as decomposers.

5. Plantae: Multicellular, photosynthetic eukaryotes that produce their own food.

6. Animalia: Multicellular, heterotrophic eukaryotes that ingest and digest food.

Note: This six-kingdom system is no longer universally accepted. Many scientists now favor a three-domain system (Bacteria, Archaea, and Eukarya) which reflects the evolutionary relationships between organisms more accurately.