By default, Malcolm is usually right, but I read a quite different explanation in a recent German encyclopedia which I paraphrase and translate in short:

ballata (ital.), balada (prov.) = dance song

In the 14th and 15th century it meant in the romanic countries a story sung in verses to a known dance tune (F. Villon, for instance).
In the 18th century, the name "ballad" in England (pars pro toto used, as so often in German, for Britain) was assigned as a new name to already existing old story telling folksongs (collect. Ramsay, Percy).

In German, "Ballade" is a (short, compared to book length) story in verses, which may be a song but very often is not. In most cases, the Ballade has started its life as poetry and a tune has been added much later by someone else. Only in recent times (last century) some songwriters write Balladen conceived as songs from the very first moment.

If you look at the titles of a German collection of Balladen you'll notice some ballads of English origin (Barbara Allen) translated into German, some ballads which are now known only as songs though they started life as poetry (Lorelei) and some old poetry which only fairly recently (200 years after it has been written) has been married to a tune (Goethe's Erlkoenig). More than half of these Balladen are still only poetry.

The influence of the English ballads upon German Balladen is seen in translation of English ballads (Edward) and in ballads that have no English precursor but are written in English ballad style and have English themes (Archibald Douglas, John Maynard, ...)

Wolfgang