Penicillin is an antibiotic (a drug used to treat bacterial infections) made from a type of mold, Penicillium, that is commonly found on bread products and is the mold that gives bleu cheese its characteristic color and flavor. Its antibiotic qualities were discovered by accident in 1928 by Alexander Fleming in a spoiled experiment, but it was not until 1942 before it was produced in large enough and pure enough quantities to be used to treat infections.
Penicillin was commonly prescribed to take care of bacterial infections in humans and animals. Since bacterial strains mutate, it's not quite as effective as it was when it first came on the market. There are now many variants of the original penicillin, and these are prescribed for various infections. All of these penicillins are beta-lactam antibiotics.
Beta-Lactam antibiotics inhibit the formation of peptidoglycan cross-links in the bacterial cell wall, eventually causing it to breakdown and the cell to burst killing it.