Classification of organisms can be explained by following categories like two kingdom, three kingdom, four kingdom, five kingdom and six kingdom.
Viruses are non cellular organisms and act as a connecting link between plants and animals. Lichens are symbiotic association between algae and fungi. Viroids are infectious agents that are smaller than virus.
Five kingdom classification was given by R.H. Whittaker. He placed monera, protista, fungi,plantae and fungi in separate groups. Monera includes bacteria, protista includes eukaryotic organisms, fungi includes eukaryotic and heterotrophic organisms, plantae includes eukaryotic and multicellular organisms, animalia includes multicellular eukaryotic heterotrophic organisms.