Prokaryotes are simple, single-celled organisms that lack a nucleus and other membrane-bound organelles. They are classified into two main groups: bacteria and archaea. Prokaryotic cells are typically smaller than eukaryotic cells and have a more straightforward structure, often consisting of a cell membrane, cell wall, and genetic material in the form of a single circular DNA molecule. These organisms are incredibly diverse and can be found in various environments, from soil to extreme conditions like hot springs. Prokaryotes play essential roles in ecosystems, including nutrient cycling and decomposition, and some are used in biotechnology for applications like fermentation and genetic engineering.