Prokaryotes are single-celled organisms without a nucleus; i.e. Bacteria and Archaea. This name comes from the Greek root karyon, meaning nut, combined with the prefix pro-, meaning before. Organisms composed of cells with a nucleus are called eukaryotes, where the prefix eu- means good or true.
Prokaryotic life is ubiquitous on Earth, present in every conceivable biome (and also being found in places where no life of any kind had been expected to exist, for example the endolithic biome). As such, there are prokaryotes with all manner of adaptations. Detailed treatments of prokaryotic structure can be found in the bacteria and archaea articles.
It is thought that the first living cells on Earth were likely prokaryotic in structure, and possibly similar to some existing archaea. Fossil prokaryotes have been found in extremely ancient rocks on Earth. There was also a recent discovery of what may have been fossil prokaryotes in a Martian meteorite, though this has since been disputed.