The word BALLAD is used for many purposes, but for us it is a particular kind of folk song. It is not always easy or even important to distinguish a ballad from other folk songs, but here are some pointers. The ballad always tells a story, usually of epic rather than homely proportions, and often tragic or even lethal for one or more of its characters. Whereas most of our folk songs come from a period between 1750 and 1850, most of the ballads are older, including one or two from the 14th century. Whereas most of the folk songs were "recovered" from oral tradition in the 19th and 20th centuries, the ballads were often in written form, in the hands of scholars, learned institutions, and landed gentry. Many ballads are to be found elsewhere in Europe, especially in Scandanavia, and have received a great deal of attention from scholars over the years. Whereas ballads have always been a respectable area of academic study, this has only been true of folk song in the later part of the twentieth century.
The definitive collection of ballads is that of Professor F J Child, of Harvard University, who published his The English and Scottish Popular Ballads in the 1880s. He decided that many of the the ballads he studied were in fact different versions of the same story, and eventually finished with 305 distinct stories, which were numbered, and have ever since been known as Child Ballads. Child Number 1 is Riddles Wisely Expounded, and Child Number 305 is The Outlaw Murray. The commonest, and best known is  Barbara Allen (Child No. 84).
Child's primary interest was literary, and he included only a few tunes in his book. However in the 1950s, another American professor, Bertrand Harris Bronson of Princeton, looked for as many of the traditional tunes as he could find, on both sides of the Atlantic, and published them as The Traditional Tunes of the Child Ballads in four splendid volumes from 1959 to 1972.