A nucleoid is the part of a prokaryote, or single-celled organism, where genetic material for the cell is contained. Prokaryotes do not contain true nuclei or even organelles. Therefore, the genetic material for the organism exists in the nucleoid, which is a region in the cell without a membrane enclosing it.
According to How Stuff Works, nucleoids exist only in the oldest of organisms: the prokaryote. This organism only has one cell, which has no nucleus or organelles. While eukaryotes have a distinct nucleus where genetic material is replicated and surrounded by a protective membrane, prokaryotes only have the nucleoid, a region in the cell where the replication takes place and genetic material is found.
The genetic material of a prokaryote is called the genophore. The genophore has neither chromatin nor telomeres to support it during replication. Instead, it is compacted through a supercoiling mechanism into a circular shape. However, prokaryotes have cell walls, much like plants, and also have ribosomes and plasmids as seen in eukaryotic cells.
Prokaryotes are prmarily either bacteria or archaeans, a bacteria-like organism that is supposedly the link between bacteria and eukaryotes. Prokaryotes can survive in very inclement conditions, such as hydrothermal vents and hot springs.