its a bar of clay (detailing clay, its more durable and tough than craft clay) and it removes deposits on the paint, making is slick as crap.
i'm a pro car detailer, and i got 2 words of advice.
a pressure washer is best choice, but for sensitive areas (sensors, wires, plugs, etc...) just stand back and no pressure will be put on those areas.
also, after putting the degreaser on for 15 minutes, put another quick coat on before you wash it off, as some of the degreaser can dry, this will aid in faster cleaning.
great post hung, alot of people don't know how to do that stuff the easy way and end up f*cking alot of crap up (liek the dizzy)