The primary characteristic of a prokaryotic cell is the lack of a membrane-bound nucleus. Prokaryotic cells, which include bacteria and archaea, are generally smaller and simpler than eukaryotic cells. Their genetic material is located in a region called the nucleoid and is usually in the form of a single circular chromosome. Additionally, prokaryotes often have a rigid cell wall, plasma membrane, and may possess structures like flagella for movement. This simplicity allows them to thrive in various environments.