Bacteria are single celled prokaryotic organisms. Prokaryotic means that they do not contain membrane-bound organelles. In simpler terms, they don't have cell parts within their cytoplasm. Bacteria are classified by their shape- round or cocci, rod or bacilli, and spiral or spirilla. Bacteria may also have long whip-like tails called flagella or be covered with small hair-like projections called cilia. Both structures are used for movement. Bacteria may have a cluster of free floating DNA in its cytoplasm called a nucleoid or they may have additional small rings of DNA called plasmids. Lastly, a bacterium have a cell wall made of a special compound called peptidoglycan which can provide an extra layer of protection from many environmental conditions.