The most common braking system on medium and larger trailers is electric drum brakes. They are controlled from the tow vehicle and are reliable and adjustable. Here is how electric trailer brakes work.
How they work
Each braked wheel has a drum brake with an electromagnet inside. When the brake controller sends current, the magnet grabs the spinning drum face, which levers the brake shoes outward against the drum - see how drum brakes work. More current means harder braking.
Controlled from the truck
The amount of braking is set by the brake controller in proportion to how hard you are stopping, so the trailer brakes blend with the tow vehicle. You can usually adjust the gain and even apply the trailer brakes manually with a lever for sway control.
Advantages
Electric brakes are simple, adjustable, and let the driver fine-tune trailer braking - and apply them independently to settle trailer sway. They are the standard on most travel trailers and heavier utility trailers.
Maintenance
Electric trailer brakes need periodic inspection of the magnets, shoes, and wiring, and the drums wear like any drum brake. Keep the tow vehicle's own brakes strong too; see best pads for towing. Fit quality pads and hardware.