Like trailers themselves, trailer brakes come in a variety of sizes and system designs. Conversely, there is at least one constant: Without proper brakes and control of them, your trailer becomes an oversized yo-yo.
Modern consumer tow vehicles - pickups or sport/utilities - are frequently called upon to tow a trailer. Horses, boats, mobile houses, utility and stakebeds, race cars, and any manner of two-stroke buzz bombs need a trailer to get to the boat ramp, campground, or track. Since many of these combinations end up weighing more than the tow vehicle, and most states and vehicle manufacturers require them on trailers over a minimal weight, trailers have brakes.
Laws regarding brakes on towed vehicles vary by state and type of trailer. Some states require brakes on any trailer or anything over a set weight (loaded), while others use the type of trailer and/or number of axles as guidelines. In many cases, any trailer over two axles must have brakes on at least two of those axles.
Tow-vehicle manufacturers have their own guidelines regarding trailer weights and brakes, with all maximum tow ratings given for braked trailers. Generally speaking, the non-braked trailer weight allowed by a manufacturer is a small fraction of the tow rating with trailer brakes, and some go as far as listing tow maximums separately for boats and other trailers because boats have much less frontal area. Further variations can be found with four-wheel drives that occasionally list towing weights for both high- and low-range operation. In Truck Trend road-test specifications, the maximum tow rating listed is for a braked trailer and normal high-range gearing. As we prefer to err on the conservative side, it's also our opinion that - manufacturer guidelines and laws notwithstanding - any multi-axle trailer or single axle that you can't move around on your own should have brakes.
The medium that supplies brake-action power on trailer brakes is air, hydraulic or electric. Commercial vehicles rely on air and hydraulic systems where the tow vehicle and trailer use the same circuit. Virtually all recreational trailers and boats use either hydraulic surge brakes or electric brakes. Regardless of system, the end result is a pressurized application of one material (the pad or shoes) against another (the drum or disc) to convert mechanical energy into heat energy. The heat energy then dissipates into the atmosphere, unless you've added wheels or fenders that limit airflow to the brakes or you've overheated them by excessive application, at which point the heat is transferred to wheel bearings, seals, and the fluid itself.