The brake light switch is a small part at the top of the brake pedal that tells the car you are braking. When it fails, the symptoms reach beyond just the lights. Here are the symptoms of a bad brake light switch.
Brake lights won't come on (or stay on)
The obvious sign is brake lights that do not light when you press the pedal - a serious safety issue - or lights that stay on all the time, draining the battery. Either points to the switch or its adjustment.
Won't shift out of park
Many automatics require the brake pedal (and the switch) to shift out of park. A failed switch can leave you unable to move the shifter - a common, confusing symptom that is actually the brake light switch.
Cruise control or other issues
The switch also tells cruise control to disengage when you brake and feeds the ABS/computer. A bad switch can disable cruise control or trigger warning lights; see brake warning lights. It is a small part with wide effects.
What to do
Brake light switches are cheap and usually easy to replace and adjust at the pedal. Fix it promptly - working brake lights are a safety must. Keep the brakes themselves fresh with quality pads and hardware.