Archaea

Archaea (or sometimes Archaebacteria) are recognized as the third domain in the taxonomy (besides Bacteria and Eukaryota). Before, they were categorized as Bacteria, for their cell structure is similar. Yet they differ in genetic processes and a lot of other stuff.
The first observed species were those in extreme conditions (e.g. high or low temperature, saline environment, high pressure), however there are as well those living in quite normal conditions.
The relations to the other domains are uncertain. Some think they may have been the ancestors of Eukaryota, some think that by "crossbreeding" Archaea and Bacteria, Eukaryota emerged, some think the three are wholly different groups coming from one common ancestor...