A normal power brake system is powered by engine vacuum, you have a vacuum line going from your intake to the big drum on your brake booster.

With a hydroboost setup instead of using vacuum to power the brake it uses hydraulic pressure, usually tapped into the power steering pump. I've been thinking of getting one for my truck. Never actually driven a vehicle with them but they're supposed to be a lot better than vacuum.

I'd definitely like to hear how this works out.