Cephalosporin is an antibiotic originally derived from the acremonium fungus. It's one of a group of antibiotics known as the cephems. It was first isolated in 1948, ironically from a fungal culture found in a sewer. The discoverer, Italian Giuseppe Brotzu, noticed the fungus killed the type of salmonella responsible for typhoid fever. After a long development process, it was first marketed as a pharmaceutical in 1964.
Like penicillin, cephalosporin works by preventing bacteria from forming a new cell wall when they duplicate. However, the mutations that make bacteria penicillin-resistant work much less well against cephalosporin, making it more likely to develop a similar resistance and it will work against penicillin resistant strains.
Like most antibiotics, the side effects of cephalosporin are mild and are generally limited to diarrhea and nausea. It can cause an allergic reaction and many people allergic to penicillin are similarly allergic to cephalosporin.