The Brotherhood of Steel is a technologically advanced faction group that wants to kill all mutants. They distrust the unwashed masses and believe that they are not to be trusted with the cool technology that they invent, preferring to keep it to themselves. However, as the Vault Dweller, you'll get a chance to get hands on with some of their tech, including their power armor.

The Brotherhood of Steel power armor is one of the coolest items you can get in Fallout 3. You'll need special training to wear it, however, which means you have to complete a good part of the main storyline before you get a chance to try it on for size. It's worth the effort. Just look at it. The only thing more intimidating is the apocalypse, and if you wanted to be any more armored you'd need to wear a tank.