Penicillin is a type of antibiotic that was discovered by Alexander Fleming in 1928. It is derived from the Penicillium mold and is primarily used to treat bacterial infections by inhibiting the growth of bacteria. Penicillin works by interfering with the formation of bacterial cell walls, leading to cell death. It has been instrumental in reducing mortality from bacterial infections and is still widely used today, though some bacteria have developed resistance to it.