To explain the characteristics of Archaea, and to explain how they survive in extremities.
Archaeans are the prokaryotic cousins to Bacteria. Separated from aforementioned cousin in 1977, the archaean organisms have been researched independently through phylogenetics. They can live in extreme conditions, such as volcanic hot springs and acidic run-off, due to the unique enzymes that they produce.
Unlike it's cousin domain, Bacteria, the Archaea domain has only recently popped up as a major group. The definitions for it's grouping are still debated, but here's the quite factual characteristics of Archaea...
For a little more than half of the 1900's, archaean organisms were classed together with bacteria. At that point in time, the visual and functional similarities between bacteria and archaea made it understandable to mix them. However, after discovering a new method in 1965, scientists were able to differentiate between bacteria and archaea via their gene sequences. This method was called phylogenetics.
It wasn't until 1977, however, that Carl Woese and George Fox classified Archaea as a group all their own. The argument for differentiating the two was that archaean organisms had unique ribosomal RNA gene sequences. After studying the phylogenetic tree of Archaea, it was found that archaean organisms have some connection to the evolution of Eukaryotic cells, but that's beyond the scope of this tutorial.
Archaea are also both autotropic (they make their own food) and heterotrophic (they consume other organisms).
Source: Wikipedia, Microbiology Textbook
Archaeans are really neat, and that's mostly because of where they thrive. Unlike bacteria and eukaryotes, archaeans thrive in the most extreme places on earth. This unique characteristic defines them as extremophiles. Here's some of the places that archaeans thrive...
So what enables them to live in such insanely extreme environments? Enzymes. Those interesting proteins that speed up or enable different kinds of chemical reactions. Archaeans produce certain types of enzymes that protect against acids like sulfur. Strangely enough, the same enzymes protect against the salinity of the salt lakes and against the extreme hots and colds.