What's the difference, cause to me they all seem like dudes in Power Armor with a hard-on for Energy Weapons.

I'll be the first to admit that I zoned out for about 60 % of Fallout 3's story line. Also on top of that not knowing much about Fallout lore to begin with so please be gentle. Ice Cream ?

The Enclave is remnants of the U.S. Government. The Brotherhood of Steel was founded by U.S. Army soldiers that broke away after learning that West-Tek was using military prisoners in genetic experiments. This is just a very brief difference between the two. If you want to read into it more here are some links.

Brotherhood of Steel:

http://fallout.wikia.com/wiki/History_of_the_Brotherhood_of_Steel

Enclave:

http://fallout.wikia.com/wiki/History_of_the_Enclave