Archaea is one of the three domains of life. It includes mainly extremophiles, which are bacteria that survive in extreme conditions like high temperature or acidity. Odds are that you've never encountered an extremophile. read more
The other two domains are bacteria and eukaryota. Eukaryotes include everything multicellular, so any organism you can observe with your eyes is a eukaryote. Bacteria are nearly everywhere, including on your skin and in your gut, but too small to see. read more