Until a few years ago the term "bacteria" was synonymous with the term "prokaryote". Then technology revealed the existence of numerous forms of prokaryotes that possess distinctive differences when compared to those prokaryotes we refer to as "bacteria". Therefore, we now have two domains of prokaryotes: the Archaea (Archaeprokaryota; Archaebacteria) and the Prokaryota (bacteria, Eubacteria, or Euprokaryota).

Prokaryotes are ubiquitous, and vary in terms of their nutritional modes, reproductive styles, habitats and so on. Some are photosynthetic, some are heterotrophic, some are decomposers, and others are parasites. Some forms are unicellular, others are colonial.

Most modern bacteria have cell walls (except the mycoplasmas), and those walls are composed of peptidoglycan. Modern bacteria (except the mycoplasmas) exhibit these body shapes: bacillus, coccus, or some form of spiral (helical, spiral, or vibrio). Those bacteria with peptidoglycan cell walls respond to the Gram_stain as either Gram_positive (Gram+) or Gram_negative (Gram-).

The coccus forms (pl: cocci) of modern bacteria are spherical. They may exist as independent unicellular forms or create chains (streptococci) or clusters (staphlococci).

Bacilli (sing: bacillus) have a rod-shaped body. These cells may exist as single units or form chains (streptobacilli) or clusters (staphlobacilli).

The spiral bacteria all have a twisted body shape. Variations of the spiral body shape include, but are not restricted to, the corkscrew and helix.