Dubstep is a genre of electronic dance music. It’s overall sound has been described as “tightly coiled productions with overwhelming bass lines and reverberant drum patterns, clipped samples, and occasional vocals”
Via Urban Dictionary:
Dubstep is music centered around bass. It draws on many musical influences. Such influences include, dub, reggae, d’n'b, jungle, UK garage, grime, techno and IDM. Usually around 140bpm in tempo, the unifying sounds within dubstep are always bass and sub-bass. The music ranges in style from melodious instrumentals to dark, techy and distorted numbers. Many UK grime mc’s also mc over dubstep tracks, although grime and dubstep should not be confused. Dubstep is usually mellow and generally has a well paced structure to each song. Grime beats are generally more disjointed, the overall production level being grittier than that in dubstep.