a)   Archaebacteria – They live in harsh habitats such as  exteme saline areas.They are called Halophiles.  They who lives in hot springs are called Thermoacidophiles.Those who lives in marshy areas are known as Methanogens .

b)Eubacteria  : also called  True bacteria

They are characterised by the presence of a rigid cell wall, and if motile, a flagellum.

Autotrophic bacteria - Cyanobacteria (also referred to as blue-green algae) have chlorophyll a similar to plants and are Photosynthetic Autotrophs. The cyanobacteria are unicellular, colonial or filamentous, freshwater/marine or terrestrial algae.

They often form blooms in polluted water bodies. They can fix nitrogen in cells called heterocysts e.g. Nostoc, Anabaena.

Chemosynthetic Bacteria : These bacteria oxidise nitrates, nitrites and ammonia and use the released energy in ATP production and help in recycling of nutrients ( eg. Pseudomonas, nitrobacter )

Heterotrophic Bacteria :Majority of them are decomposers.They are helpful in  making curd from milk, antibiotics, nitrogen fixing  (Rhizobium ) .Some are Pathogenic causing damage to humans by causing diseases cholera, T.B, diarrhea. They also cause damage to crops, farm animals and pets.

Bacteria reproduce mainly by fission .Sometimes, under unfavourable conditions, they produce spores. They also reproduce by a sort of sexual reproduction by adopting a primitive type of DNA transfer from one bacterium to the other.

c)Mycoplasma: They are the  smallest living cell known. They can survive without oxygen.They lack cell wall.Many are pathogenic  in animals and plants.