What is meant by gram positive bacteria and gram negative bacteria?

Staining technique. Gram positive organisms have a high amount of material in their cell walls which retain gentian violet (a purple stain) on slides looked at under the microscope. Gram negative organisms have the gentian violet washed out when subjected to acid-alcohol and then they are restained with a red stain which renders them red under the microscope. It is one way of loosely classifying bacteria.