Mechanical to Hydraulic lifters

Solid roller uses solid lifters with rollers on the cam lobe.
Hydraulic roller uses hydraulic lifters also with rollers on the cam lobe.
Both are roller lifters but the solid lifters need to have a clearance (approx .015 - .020) at the tip of the valve stem to allow for expansion, the hydraulic setup does not need any clearance as the hydraulic lifter builds oil pressure to take up slack in the drive train. Just google (images) each lifter style and you will see the difference.
Hope this makes sense.

As for the performance, I have to agree this thing rocks. But I drive my car a lot and with 4:10's on the highway the motor gets annoying.