Folk and traditional ballad go back centuries. The earliest known ballad is from the 13th century. Literary ballads go back to the 17th century, at least. Lyrical ballads, however, were popularized, if not invented, by William Wordsworth and Samuel Taylor Coleridge in 1798.
When their volume of poems was first published, contemporaries, who had notions that True Poetry (TM) could only be written in high, affected language and only be about the aristocracy or heroes, panned it. The language Wordsworth and Coleridge employed was simple, easy to understand, and (horrors!) they wrote about "unworthy" (common) people. Thus, Wordsworth published the volume again in 1800 with his now infamous preface, in which he addresses their detractors, giving his opinion of what "poetry" really is--or ought to be, in his opinion.
Essentially, whereas poetry had commonly been highly affected (For example, "Bacchus from his glassy prison free!" might be used instead of "Open the bottle of wine"), Wordsworth and Coleridge eschewed complex sentence structure and obscure references in lieu of common, straightforward language. For example:
She dwelt among the untrodden ways
Beside the springs of Dove,
A maid whom there were none to praise
And very few to love.