What do you mean by archaebacteria?

Hint: There are six kingdoms in which all organisms are classified. These are plants, fungi, animals, protists, eubacteria and archaebacteria. Archaebacteria are the bacteria found in harsh conditions. They have the ability to withstand and live in very severe environments. They are single-celled organisms and come under prokaryotes. They are primitive microorganisms with no cell nucleus.

Complete answer:
Archaebacteria have evolved separately from blue green algae and bacteria. Three major groups of Archaebacteria are present, which are thermoacidophiles, halophiles and methanogens. They are also known to be extremophiles, as they survive extreme environments. Some of the features overlap with those of eukaryotes.
Binary fusion is the main mode of reproduction. They are able to withstand pressure upto\[200\]atm. They are divergent from both prokaryotes and eukaryotes. They are able to survive the temperature in boiling points. They can live in acid and alkaline aquatic regions. They grow well in the absence of oxygen and they are able to go under the process of methanogenesis. Lipids constitute the cell membrane of archaebacteria. They resemble bacteria when viewed under a microscope. They are one of the oldest living organisms on earth.
Archaebacteria are able to perform gene flow across the species. They are methanogens as they decompose the organic matter and produce methane, which can be used for cooking. Thus, archaebacteria serve as primary producers.
On the basis of phylogenetic relationship, archaebacteria are classified into crenarchaeota, euryarchaeota, korarchaeota, thaumarchaeota and nanoarchaeota.

Note:
Carl Woese discovered archaebacteria in 1978. Unlike bacteria, archaebacteria do not produce spores. It reproduces asexually. It is not able to perform photosynthesis. In the ocean, \[20\]% of microbial cells is archaebacteria. Some archaebacteria are lithotrophs, as they obtain energy from inorganic compounds like ammonia or sulphur. Some of them are phototrophs.