While most jug bands of today use kazoos, jugs, washboards, or washtub basses I believe that "jug band" music has come to represent a certain type of music and style. Most Tunes performed by Jim Kweskin, Dan Hicks, Geoff and Maria Muldaur, R. Crumb and his Cheap Suit Serendanders, and many early Greatful Dead songs all fall into this swinging, happy go lucky, genre. It makes you click your fingers and puts a smile on your face.
I know it when I hear it, but can't put it into words.
See the threads on Skiffle for more information about what it is.