Block huggers are usually little short headders that tuck in close to the block as they drop down. They are great for tight installs like squeezing a V8 between the rails of a hot.
The disadvantage of them is their primary length is very short so there's not the usual advantages in flow that you get from a tuned length or even a tri-Y design header. They often don't perform much better than the stock manifolds these days.
Still a good option for a custom engine install where max power isn't the goal.
