"Gram stain" is a staining dye named after Dr. Gram. Some bacteria can be stained by this and be made visible, while others are not. If they are they're called "gram posisitve" (stained by gram's stain) and if they are not they're "gram negative" (not stained by gram's stain).

This is very important as there are two general classes of antibiotics, you guessed it, one class for gram-posiitve bacteria and one for gram-negative bacteria.

This seemed to me like a very broad categorization,and I never understood why until altum at mad dor scientist.com gave this explanation on usenet:

Gram staining not a broad generalization at all. The staining is very specific to bacteria with a thick aminoglycoside cell wall. It is easy to perform and very reproducible, which is why the method is still extensively used. Antibiotics like penicillin inhibit synthesis of this wall and cause *only* gram positive bacteria to lyse. The cell wall also changes the permability of many antibiotics. The label "gram negative" does encompass a very wide variety of organisms, and knowing that an organism is gram negative doesn't tell you much.