Brakes dust. When you apply them the charged particles stick ot your rims, your rotors and your pads. Most pads are slotted and champfered and people think that is for heat, which it is but the slots alos help channel any dust away from the pad surface.
For the rotors you can use CRC or Kleanflo Breakclean BUT don't get it on any painted surfaces and don't get it on the pads. The pads usually clean up ok with water but if you want to do a thurough job takes the pads off and use a soft metal bristeld brush to LIGHTLY rescuff the surface of the pads and get rid of that compacted burnt brake dust. If you dont have a metal bristle brush and I don't mean steal but bronze brushes you can get from welding supply stores you can use those nylon brushes that have stiff bristles and just brush the pads off. You just want to lightly do it. You can ruin pads this way as well.
Rotors should have a slight ruffness to them and not be polished. If they are polished your brakes will squeal and you need them resurfaced. If they arent warped you can do that with a 3M Red Roloc disk. You wont be removing any metal but you will be scratching the surface of the rotor and getting rid of any glazing that has appeared.
This is all extreme scenarios and after you have done any of this you need to let your brakes break in again for a little bit. You cant mess around with brakes.
After a good ride let your bike cool right down and use a little water to wash away the brake dust.
When you replace your pads make sure you go to a ceramic based compound. It lowers the dust, increases the stopping power, decreases the break in time and last longer.
It's brakes so don't go extreme with cleaning cuz you dont want to jeapordize the integrity of the pads so just go easy less is more here.
There are also break in sprays from Kleen Flo that help in the break in period and also cut down on glaszing which is one of the causes of squeals. That stuff you follow the directions. I cant remember if it goes on the pad of the rotor or both.
http://www.kleenflo.com/en/index.htm
Also make sure your caliper pins are lubed up with a brake specific lubricant.