A "B-boy" or "B-girl" is a person devoted to hip hop culture, more specifically, "bboying / breakdancing".
There are four basic elements to B-boying:
'Toprock' - the upright dancing and shuffles that B-boys do when they enter a circle.
The 'Downrock' or 'Footwork' - dancing performed on the floor.
The 'Freeze' - the poses that B-boys throw into their dance sets to add punctuation to certain beats and end their routines.
The 'Power' moves - acrobatic moves normally made up of circular motions where the dancer will spin either on the floor, or in the air.