This is the start of a six page definition from Funk and Wagnall's Standard Dictionary of Folklore - fairly comprehensive, but doesn't suit everybody, especially those who don't hold withnew-fangled gadgets like dictionaries
Jim Carroll.

Ballad - A form of narrative folk song, developed in the Middle Ages in Europe, to which has been applied very ambiguously the name ballad (Danish vise, Spanish romance, Russian bylina, Ukrainian dumi, Serbian junacka pesme, etc.). This type of folk song varies considerably with time and place, but certain characteristics remain fairly constant and seemingly fundamental: 1) A ballad is narrative. 2) A ballad is sung. 3) A ballad belongs to the folk in content, style, and designation. 4) A ballad focuses on a single incident. 5) A ballad is impersonal, the action moving of itself by dialog and incident quickly to the end.
A ballad is story. Of the four elements common to all narrative—action, character, setting, and theme—the ballad emphasizes the first. Setting is casual; theme is often implied; characters are usually types and even when more individual are undeveloped, but action carries the interest. The action is usually highly dramatic, often startling and all the more impressive because it is unrelieved. The ballad practices a rigid economy in relating the action; incidents antecedent to the climax are often omitted, as are explanatory and motivating details. The action is usually of a plot sort and the plot often reduced to the moment of climax; that is, of the unstable situation and the resolution which constitutes plot, the ballad often concentrates on the resolution leaving the listener to supply details and antecedent material.
Almost without exception ballads were sung; often they were accompanied by instrumental music. The tunes are traditional and probably as old as the words, but of the two—story and melody—story is basic. Many ballads were sung to a variety of melodies. Unlike lyric songs in which the meaning is not so important and which are consequently subordinated to the music, ballads, in which the contrary situation obtains, always subordinate the melody to the words. More variety exists in ballad music than in ballad form and content, for it ranges from the modal types of the West, based on the Gregorian, to the more florid and ornamental types of Greece, the Balkans, and Russia owing much to Byzantine tradition. Here and there, as for example among the South Slavs, instead of melody the ballad is often accompanied by rhythmical chant, almost recitative. The point is, of course, that the ballad is not simply recited or told, but given interpretation and emotional power by the accompanying melody.
The ballad belongs to the folk, but it is by no means primitive or barbaric; rather it is the product of accomplished and often literary-conscious poets.