Ballads are an increasingly common musical concept in modern popular music. They're generally thought of as slow, highly melodic songs that tell of some sort of heartbreak; there's even a whole CD dedicated to power ballads, the result of heavier rock bands taking it down a few notches to tell about love gained or lost. But true ballads, in the strictest sense of the word, are far more complicated than just taking a song's tempo down by a few beats per minute. True ballads date back to way before heavy metal or modern folk music. True ballads are an integral part of historical folklore.
Ballads are songs that tell a story, like poetry or folk stories set to music. The stories told in ballads tend to be well-known and oft-repeated and often incorporate tall tales. Increasingly, however, ballads have been adapted to the plight of the everyman; ballads are the peoples' music and will always tell about the people of that particular time.
Ballads are distinguishable from other types of songs by a few important characteristics. The lyrics in ballads tend to focus far more on action than reaction and will usually be sung in the third person, like many literary folk stories. Ballads also tend to focus on the song's lyrical (as opposed to instrumental) qualities; since a ballad is focused so heavily on the story being told, it's vital that the lyrics get first billing. Because of this, the lyrics tend to be simple and easy to repeat. The chorus, especially, must be memorable; it was designed specifically to be sung in unison by the audience, many of who will have only heard it once. This memorable quality is vital also because ballads, in the truest sense of the word, will almost always have been passed down from generation to generation. The ballads will certainly evolve over time, but the basic elements of story will remain in tact.
Because true ballads tell folk stories, it's not surprising that ballads have remained a large part of the folk music genre. But even in that most integrity-based area of music, ballads haven't always maintained their essential elements. The notion of ballads, just like the ballads themselves, has evolved so much over time that the ballads of today barely resemble those of the original criteria. Still, despite their differences, they are still considered ballads -- just in a much looser form (consider the modern ballad "American Pie"). Power ballads, on the other hand, can hardly be called ballads, even by those who allow the word to be very loosely interpreted. Power ballads rarely fit any of the criteria of original ballads, other than the frequently found subject of a lover scorned. No matter how slow, melodic or heartbreaking certain songs are, they can't always be called ballads; a catchy chorus does not always a ballad make.