By law any trailer over 16 foot in length is required to have some form of braking system.  The most common is an electronic braking system which uses 12 volt power from the tow vehicle to magnetize brake pads installed on the trailer.  A trailer brake control box is installed inside the vehicle to determine when and how much power to send out to the trailer.  Electronic trailer brake controllers can be broken down into two main styles.

As the brakes on the tow vehicle are applied a signal is sent from the brake controller to the brakes on the trailer.  The longer brakes are applied the stronger that signal gets.  Most brake boxes work this way but you DO NOT want this type because if you slam on the brakes the trailer brakes reacts very little and if you gradually ease up to a red light the trailer brakes will be locked down by the time you get there. 

Proportional electronic brake controllers have built in inertia sensors that determine the tow vehicles rate of deceleration and apply an equal amount to the trailer brakes so both work together for a smooth braking response.  This is a much better choice than time delay and most proportional boxes are extremely easy to setup and operate eliminating the user error that causes many controllers not function properly.