recently i got a new job and i got new radeon 5850 which supports dx11 so i stated to check out those nastly new dx11 features, and let me tell you that: hw tessellation is COOOOL.
little info, dx11 tessellation works with 3 new stages added:
1.) the hull shader (aka HS) - get [1..32] input control points of the patch and responsible to output [1..32] new control points, it is also output constants which control how much to subdivide each patch.
2) the tessellator (fixed-function) - get the output of the hull shader as input and generate the new points, you have no control on this stage.
3) the domain shader (aka DS) - generates surface geometry from the transformed control points from a hull shader and the UV coordinates. the DS run once for each point generated in step 2, the input of DS is bary coords of the point on the patch so you could easily interpolate vectors other than position and uv.
HS supports 4 different partitioning methods, topology and few more things (check out dx sdk if you are interested on the tiny details)
so why do we need it if we already have geometry shader? well, geometry shader is programmable and generic in a way that you can do whatever you want in it, such has point sprites, fur, object silhouette and more but its not free and not optimized for things like tessellation. in dx11 you have dedicated unit for doing tessellation so the speed can't even compared with gs.
here is a video that shows dx11 tessellation with adaptive tessellation: