There are three principal forms of bacteria. Spherical bacteria are termed cocci. Rod-shaped bacteria are called bacilli. Spiral bacteria are rigid (spirilla), flexible (spirochetes), or curved (vibrios).

The main reaction that happens during a gram stain is the detection of the presence of peptidoglycan. Most bacterial cell walls contain peptidoglycan, but in varying amounts. If the cell contains large amounts of peptidoglycan, it will retain the crystal violet dye and display a purple coloration (this is gram-positive). If the cell does not contain a large quantity of peptidoglycan, it will display the counterstain and appear pink (this is gram-negative).

The gram-stain does not dictate whether or not a species belongs to the domain archaea. Archaea cell walls do not contain peptidoglycan and will not react to the stain. Note that archaea and bacteria are different domains altogether, though they share some similar characteristics.

The peptidoglycan layer of bacteria retains the Gram stain. In general, Gram-negative bacteria are more virulent than Gram-positive bacteria. This is due in part to an outer layer of lipopolysaccharides present on Gram-negative bacteria that invokes immune responses (particularly via a component called lipid-A), and are somewhat more difficult to target with antibiotics.

Streptococcus, the causative agent of strep throat, is a genus of gram-postitive (purple staining) cocci (spheres), in which the cells tend to form chains. Bacteria are described by a combination of their response to gram stain and their appearance. For example, "gram-positive streptococcus" indicate chains of spheres that stain purple.