What are gram negative and positive bacteria?
The Gram stain is one of a large number of techniques used to characterize bacteria. In particular, a bacterial species is usually either Gram-positive (purple when stained) or Gram-negative (pink when stained). In Gram-postive bacteria, the cell wall is thicker and has much more peptidoglycan compared to Gram-negative bacteria. The chemicals in the stain react with these molecules in the cell wall and result in the purple color. All of this information is well covered in the Wikipedia article on the subject.