The document discusses the anti-lock braking system (ABS). It begins by introducing brakes and their two main types - drum and disc brakes. ABS was first developed for aircraft in 1929 and helps prevent wheel lockup during emergency stops. The key components of ABS are a hydraulic control unit, brake modules, and wheel sensors. ABS works by constantly monitoring wheel speed and controlling hydraulic pressure to each wheel. It provides safety and steering control benefits during emergency stops. While improving braking on slippery surfaces, ABS also has drawbacks including higher costs and maintenance requirements.