By the simplest definition it is "a song that tells a story."
Many traditional ballads may be ancient and full of boring detail but there are also more recently written ones that I love. Marty Robbins "Gunfighter Ballads" told some great stories, some were old, some were more recent and some were written by Marty himself. Tom T. Hall wrote some great "story songs" in the first person, and I would call them ballads as well. Johnny Cash's "Dorrain" was also a great one in the first person as well. On the other hand I have heard mushy love songs referred to as ballads although they contained little if any story line. If a song tells a good story and the story, not the music, is what first catches your attention I would call it a ballad.