The great folklorist Francis James Child defined what he called the “popular ballad” as a form of ancient folk poetry, composed anonymously within the oral tradition, bearing the clear stamp of the preliterate peoples of the British Isles. Ballads, which are stories in narrative verse, are related to folktales, romances, and sagas, with which they sometimes share themes, plots, and characters (such as Robin Hood). No one knows how old the oldest are. It’s believed that they are ancient indeed -- and yet we have few historical records of them older than the sixteenth century. Little is known for certain about how the oldest ballads would have been performed -- but most likely they were recited, chanted, or sung without instrumentation. Right up to the twentieth century, ballads were traditionally sung a cappella, although today it is common to hear them accompanied by harp, guitar, fiddle, and other instruments.
Why do we have so few historical records? Because until relatively recently, they weren’t considered important enough to write down. With the rise of literacy, the songs and poems of Britian’s great oral tradition began to fall out of favor -- and ballads that had once been popular among all classes of society were now deemed primitive, pagan, the province of unlettered country folk. Because of this, few attempts were made to preserve ballads prior to the seventeenth century, and thus many were lost or were passed down through the years in fragmentary form. In the eighteenth century, ballad collection was still haphazard and sporadic, and the fruits of such labor were little regarded in academic circles. Universities did not yet consider folklore a respectable area of study, so manuscript collections remained in private hands, easily lost and forgotten.
In 1765, Bishop Thomas Percy came across one manuscript full of fine old ballads being used to light a kitchen fire. He saved them from the flames and published them in his book, Reliques of Ancient English Poetry. Percy’s book was a great success. It was much admired by such English Romantic writers as Coleridge, Southey, Shelley, and Keats, as well as the German Romantics Goethe, Tieck, and Novalis, and sparked much literary interest in the songs and legends of bygone days. Another fan of Percy’s book was the novelist Sir Walter Scott, who collected the ballads of his native Scotland in the early nineteenth century. Scott sat at the center of a circle of poets and antiquarians who were devotees (and romanticizers) of the ancient history of the British Isles. This group did much to popularize the old songs and tales of Scotland, England, and Ireland -- but still no British university would sponsor a proper academic collection of the country’s ballads.
That job fell to an American scholar, Francis James Child of Harvard University, who was urged to take on the subject by his frustrated British colleagues. Child hesitated, somewhat daunted by the immensity of the job at hand, and then he plunged in, devoting the rest of his life to the study of ballads. Beginning in the 1870s, Child set out to track down every extant version of every genuine popular ballad in the English and Scottish traditions. He limited himself to England and Scotland because the ballads of these countries overlapped, whereas Irish ballads were a separate tradition, requiring a depth of knowledge of Ireland’s language and history he didn’t possess. His goal was to publish the collected ballads with notes tracing their histories, relating them to songs and tales to be found in folklore the world over. The result of this remarkable labor was The English and Scottish Popular Ballads, published in five volumes between 1882 and 1898. It’s a work that’s still widely used today, revered by scholars and musicians alike.
The life of the man behind these famous books is as interesting as the ballads he loved. Born the son of a sailmaker, Child grew up on the docks of Boston harbor -- until his aptitude for learning brought him to the attention of a distinguished Cambridge scholar. The boy was encouraged to transfer from his working-class school to Boston’s Latin School, after which he was sponsored at Harvard, where he graduated at the top of his class. Except for two years of study abroad, Child spent the rest of his life at Harvard, rising to become the first chairman of the newly created department of English. He built his substantial reputation on groundbreaking studies of Chaucer and Spenser, but he also had an abiding love for philology, ancient poetry, folklore, and fairy tales. The latter interests had been whetted during the two years Child spent in Germany, where he’d been exposed to the work of the folklore enthusiasts of the Heidelberg Circle of scholars, which included folk song collectors Clemens Brentano and Achim von Arnim, and the remarkable Brothers Grimm. The English and Scottish Popular Ballads, noted Child’s friend and colleague G. L. Kittredge, “may even, in a very real sense, be regarded as the fruit of these years in Germany. Throughout his life he kept pictures of Wilhelm and Jacob Grimm on the mantel over his study fireplace.”
Child was a textual scholar rather than a field collector, and he put his massive ballad compilation together by seeking out every manuscript copy of ballad material he could lay his hands on, with the help of a small army of fellow scholars searching out songs and fragments of songs throughout the British Isles. Another reason he depended on manuscripts rather than the memories of folk 
musicians was that the British popular ballad, in his view, was no longer a living tradition. The ballads he sought were the ancient ones -- not the “broadside ballads” that dominated the nineteenth-century folk musician’s repertoire. Broadsheet ballads were authored song lyrics designed to fit traditional tunes, cheaply printed and sold for pennies on street corners from the sixteenth century onward. These were contemporary compositions, rather than ancient poetry from the oral tradition -- though sometimes broadside ballads mimicked the language of much older songs, and determining which was which was a problem Professor Child was both intrigued and vexed by.
To the dismay of this meticulous scholar, in the absence of clear historical records he was often forced to depend on textual clues and his own best judgment. Fortunately, that judgment was finely honed by his fluency in archaic languages, and his extraordinary knowledge of folklore traditions the world over. He chose, he explained in a letter to a friend, to err on the side of inclusiveness. Where he had lingering doubts about the authenticity of a song variant, he was apt to 
include it anyway, along with notes outlining his reservations. His task was greatly complicated by the fact that the ballads of Britain had been so badly recorded and preserved compared with those of other countries, such as Denmark. “The ballads should have been collected as early as 1600,” he noted sadly; “then there would have been such a nice crop; the aftermath is very weedy.” Another complication was that ballads written down and published from the eighteenth century onward had been edited, censored, or “improved” by folklore enthusiasts who were literary men, romantics rather than rigorous academics. The prime example of this was Percy’s famous Reliques of Ancient English Poetry. Child and other folklorists suspected that Percy had altered the text of ballads to suit the literary tastes of his day -- particularly as Percy would not allow an examination of the ballad manuscript in his possession. Working with British scholar F. J. Furnivall, Child was instrumental in persuading Percy’s descendants to finally release this manuscript, which did ideed confirm that Percy had edited and “improved” the original ballads.
Sifting through the mountain of material he collected, sniffing out alterations and forgeries, Child amassed a group of 305 songs with their roots in the oral tradition, along with variants of each song, sometimes in dozens of alternate versions. The final volume of The English and Scottish Popular Ballads was completed the 
year of Child’s death, but he died before writing the book’s introduction, which would have explained his method of selection and given us an overview of his work. Yet even without this, The English and Scottish Popular Ballads was hailed by critics on both sides of the Atlantic and became a cornerstone of modern folklore scholarship. In addition, Child was instrumental in establishing the American Folklore Society, serving as its first president from 1888 to 1889. But sadly, Child did not live to see that movement flower in subsequent years, and he died doubting his work had relevance to a modern age. “If he’d lived just a little longer,” says Mark F. Heiman of Loomis House, which published a handsome new edition of The English and Scottish Popular Ballads, “he would have seen the golden age of the ballad collector and folklorist. He would have seen how important his life’s work really was.”
Child’s work went on to inspire a whole new generation of folklorists, men and women who weren’t quite so convinced that the oral tradition was irretrievably dead and gone. One of them was Cecil Sharp, who began collecting English folk songs and dance tunes in the early years of the twentieth century. Sharp was a trained musician, and unlike Child he was also interested in preserving the music of the ballad tradition rather than viewing ballads primarily as poetry. He noted that the Child ballads were rarely part of the repertoire of the elderly singers he listened to in the countryside; they’d been replaced by broadside ballads and other more recent songs. Sharp wondered if the older ballads might have survived among the British and Scottish settlers in America, particularly among the descendants of settlers in isolated mountain regions, where “pennysheets” of modern ballads would not have been available. Between 1914 and 1918, Sharp made two extensive trips through the Appalachian Mountains, collecting over a thousand songs with the aid of his secretary, Maud Karpeles. Sharp and Karpeles discovered that many of the Child ballads were indeed still known and performed in Appalachia, although sometimes the titles and lyrics had changed somewhat in this new setting. Sharp published these ballads in his now-classic English Folk Songs from the Southern Appalachians, which in turn inspired new folklore studies and new collection efforts throughout the United States.
Despite the keen interest of folklorists, ballads remained a specialized interest for much of the twentieth century, until the huge folk music revival of the 1960s and ’70s. In those years, Joan Baez, Judy Collins, and other popular singers recorded ballads from the Child collections, and a Celtic music revival exploded across the British Isles, Brittany, and America. Folk-rock bands like Pentangle, Fairport Convention, and Steeleye Span updated the ballads for a new generation, while singers like Martin Carthy, Nic Jones, Frankie Armstrong, Jean Redpath, and June Tabor created an audience for traditional music played in more traditional ways.
Today, that revival is still going strong, with Child ballads performed by Jon Boden, Iona Fyfe, Fay Hield, Sam Lee, Malinky, Loreena McKennit, Jim Moray, Karine Polwart, Kate Rusby, and many, many others. (You'll find an online discography here). I particularly recommend Anaïs Mitchell & Jefferson Hamer's Child Ballads album, and Jon Boden's Folk Song A Day site. To dig further into this subject, you'll find a lot of good material in the digital archives of the English Folk Song & Dance Society. To read about the ways the Child ballads have influenced fantasy literature and comics, go here.