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What are bacteria?

Bacteria are prokaryotic and unicellular beings. Bacteria have simple organization, they present an external cell wall, plasma membrane, circular DNA within the cytoplasm and ribosomes for protein synthesis. Some bacteria are encapsulated, i.e., they have a polysaccharide capsule outside the cell wall.

Are bacteria the only prokaryotic beings?

Prokaryotic beings are classified into two big groups: archaebacteria and bacteria (this last also known as eubacteria).

Comparing to bacteria, archaebacteria have basic differences, like the chemical compositions of their plasma membrane and cell wall and different enzymes related to DNA and RNA metabolism.

What are halophile, thermoacidophile and methanogen archaebacteria?

There are three peculiar types of archaebacteria. The halophile archaebacteria only survive in salt-rich environments (even salinity of the sea is not enough for them). Thermoacidophile archaebacteria characterize for living under high temperatures and low pH. The methanogen archaebacteria are those that liberate methane gas (CH4), they are found in swamps.