What is the definition or description of: gram stain?

For bacteria: The two major families of common bacteria are distinguished by the chemistry of their walls, specifically the ability of iodine to mordant their staining with crystal violet and prevent its being removed with strong solvent. Gram positives include staph and strep, while gram negatives include e. Coli. The stain has been in use for around 150 years and remains the most popular in bacteriology.