Answer: Down force is the force that the air applies to the car as the car goes thru the air giving the car more traction, by pushing the car down. The down force allows higher cornering speeds,improved braking/ traction. It Increases with speed, most aerodynamic aids really only start working( give any significant help) when the vehicle is going 75/100mph+. One downside of wings/etc. they always cause an increase in drag( only exception is a sealed undertray). This increace in drag affects top speed and fuel economy. An example the last viper gts could do 200-202 mph, but the acr with the downforce aids, top speed drops down to around 170-175mph.
Answer: Generally the front brakes of an average type braking system stops approx. 60% or more as opposed to the rear brakes wich attribute to 30-40% of the total braking power on said vehicle, that is why typically the front brake master cylinder reservoir compatment contains twice the amount of fluid in it as the rear brake master cylinder fluid fill compartment.
Answer: Hydraulic psi applied to the brake pads, convert the vehicles Kinetic energy into (thermal energy)heat. That is why race cars have brakes that can generate more heat(vented thick steel or carbon fiber) and are lighter(less kinetic energy).