Short for Discharge beat or down beat (depeneding on who you ask), whatever the technical definition, d-beat still refers to the same thing, which is shredding guitar, sparsed vocals (often about the destruction of mankind/the earth) and the d-beat.
D-beat bands include Discharge, Disclose, Anti-Cimex, and Wolfbrigade. D-beat is also closely associated with Swedish, Finnish, and Japanese Hardcore bands such as Gauze, The Black Uniforms, and Tampere SS.
D-beat fashion is all black, with band shirts that printed with white lettering. Black charged haircuts, black stretch jeans, multiple studded belts topped off with a bullet belt is pretty standard on a d-beat kid. Also, headbands of various types are often worn(usually to help keep up the charged haircut).