Back up a step. Most common brakes on your average runabout boat trailer, big jet ski trailer or utility trailer (...if they have any brakes...), camper trailer, etc. are surge, not electric. There is no general electric control for that type of brakes--the trailer connector then only runs the lighting. The only exception is if a reverse signal is used to defeat the surge slave cylinder when you back up.
This common set up means they have a slave master cylinder built into the trailer tongue. The force of the tongue against the vehicle hitch as you slow down then pushes the trailer tongue parts together and puts pressure on the master cylinder inside of it to activate the brakes. The trailer has a whole separate hydraulic system that runs back to the wheels and either drum or disc brakes there. If you ever see a trailer with a boxy looking assembly about a foot long and four our so inches high bolted or welded just behind where you make the ball attachment with the tongue, that's the enclosure for the slave cylinder. If there is no such box, it either has electric brakes, or if a small trailer, likely no brakes.
Yes, there are electrically actuated brakes too, but I have typically run into/seen those either on bigger or older trailers.
From a 5 second Google search, here's a backgrounder blurb on the difference: http://www.redtrailers.com/ShowArticle.asp?id=2