Many possible definitions. In popular music, say 1920-60 and beyond, a sonbg with a thirty-two structure(probably with a preceding verse). The 32 bar bit should have an AABA structure. The B 8 bars are referred to as the middle 8, and often change key, or have a markedly different feel to the rest. Blue Moon, Sunny Side of the Street wouls be two classic examples.
    In folky(Brit) cirecles, generally an old narratiove song. Often with a four-line verse structure with4,3,4,3 feet. (This is known as ballad meter).
eg(from Chevy Chase)
Of Witherington needs must I wail
As one in doleful dumps
For when his legs were smitten off
He fought upon the stumps.

The structural aspects have been forgotten in popular music recently, and usage tended towards any romantic not very fast song. I think there are several kinds of ballad in Irish usage, including the British usages here, but also specifically Irisah ones mentioned earlier.
   As to whether ballads have(or had) choruses: the old ballad metre songs often had the second and fourth lines as recurrent refrains(give the singer time to think up the next bit?)