Gram's Method is a staining procedure that separates bacteria into those that stain (positive) and those that don't (negative). Gram-positive bugs cause such lovely things as scarlet fever, tetanus, and anthrax, while some of the gramme negs can give you cholera, plague, and the clap. This is significant to the microbiologist and the pathologist; otherwise I wouldn't worry. Still, knowing the specifics (toss in anaerobes and aerobes as well), you can impress real medical professionals with your knowledge of the secret, arcane language of medicine.