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Archaeans are single-celled creatures that join bacteria to make up a category of life called the Prokaryotes (pro-carry-oats). Prokaryotes' genetic material, or DNA, is not enclosed in a central cellular compartment called the nucleus. Bacteria and archaea are the only prokaryotes. All other life forms are Eukaryotes (you-carry-oats), creatures whose cells have nuclei. (Note: viruses are not considered true cells, so they don't fit into either of these categories.)
However, while archaeans resemble bacteria and have some genes that are similar to bacterial genes, they also contain other genes that are more like what you'd find in eukaryotes. Furthermore, they have some genes that aren't like any found in anything else.
For more details about the differences between bacteria and archaea, see this website. (Note: this site contains some pretty technical information and language.)