I've kinda wondered the same thing, Marion. I mean, I kinda knew what was meant by a pop "ballad", but couldn't really give a definition. Wikipedia says kinda what I thought though...

"In the 20th Century, "ballad" took on the meaning of a popular song "especially of a romantic or sentimental nature" (American Heritage Dictionary). {...} often divide songs into two categories: "ballads" (slower or sentimental songs) and "up" tunes (faster or happier songs)."