Serif fonts are the oldest type of fonts in Europe. They are usually divided into Oldstyle, Transitional, Modern, Slab serif and Flair serif although there are many other classifications in existence.
Their main feature are the small "protrusions" at the ends of each stem. It is thought that they were developed when masons chiselled letters in stone and serifs were made first perpendicularly to stems to indicate where the ends of the stems should be.