Symptoms of warped/distorted discs are generally a pulsating lever / pedal but this depends if you have fixed or floating calipers.
Generally if you're discs are warped within the floating calipers maximum movement you may feel the lever / pedal pulsate a little but any more than that will result in you loosing the brake because the disc pushes the pistons back. If you have fixed calipers warped discs symptoms are more apparent but it depends on how warped the discs are.
There are many causes (feel free to add your disc woes) but main ones are driving off with a disc lock , siezed calipers / pistons, bad fitment, obviously they can also be warped in an accident or while you're bike is parked say when someone reverses onto you're front wheel.
Riding with warped discs reduces braking capabilities and in some cases you can loose the brake all together.
