Early on in one's dancing studies, I think it is a very good idea to understand some basic differences in the above 3 classes of partnership dancing. 

By "popular" dance, is meant, the freeform social dancing that one might run into at clubs and informal dances. This class of dance can be as loose as fad dances (freestyle dancing as covered in this Wikipedia article: http://en.wikipedia.org/wiki/Novelty_and_fad_dances); Salsa, Bachata, Merengue and Cumbia in the Latin-influenced dance scene; Blues, Swing, Lindy, Balboa and the numerous variations in the Swing/Blues scene; Argentine Tango (for a more authentic and rhythmical tango-style); line dances; and on and on.