Short answers, pulled from the story.
Archaea are a domain of organisms, distinct from Bacteria and Eukaryota. Their cells have unique properties including cell membranes made of ether-linked lipids, metabolisms such as methanogenesis, and a motility structure called an archaellum. Unlike bacteria, they lack peptidoglycan in their cell walls.
Archaea were first classified separately from bacteria in 1977 by Carl Woese and George E. Fox, based on their ribosomal RNA genes. At that time only the methanogens were known. Woese, Otto Kandler, and Mark Wheelis later proposed the three-domain system of Eukarya, Bacteria, and Archaea, now called the Woesian Revolution.
The word archaea comes from the Ancient Greek for ancient things, because the first representatives of the domain were methanogens. It was assumed their metabolism reflected Earth's primitive atmosphere and the organisms' antiquity. The archaeal lineage may be the most ancient that exists on Earth.
Archaea exist in a broad range of habitats, from hot springs, black smokers, and oil wells to cold polar seas, salt lakes, soils, sewage, and the intestinal tracts of animals. The first-discovered archaea were extremophiles, but improved detection later found them in almost every habitat. They may represent about 20 percent of microbial cells in the oceans.
Archaea look like prokaryotes in structure but are genetically more similar to eukaryotes, sharing most genes involved in gene expression, transcription, and translation. Archaea and Eukarya share a more recent common ancestor than either does with Bacteria. A lineage called Lokiarchaeum, discovered in 2015, was the closest known archaeal relative of eukaryotes at the time.
Extremophile archaea supply heat-stable enzymes, such as the Pfu DNA polymerase from Pyrococcus furiosus that made PCR a routine technique. Enzymes from Pyrococcus that work above 100 degrees Celsius enable food processing like low-lactose milk and whey. Methanogenic archaea are vital to sewage treatment and biogas production, and acidophilic archaea show promise for extracting gold, cobalt, and copper from ores.
There are no clear examples of known archaeal pathogens, and as of 2024 only one species of non-eukaryotic archaea has been found to be parasitic. Many archaea are mutualists or commensals, such as the methanogen Methanobrevibacter smithii, which makes up about one in ten of the prokaryotes in the human gut and may aid digestion.