Set of 16 lifters in each box.

Roller Lifters have a roller on the end that rides on the cam shaft. This reduces valve train friction and allows for a power increase. Roller Lifters also allow for greater lifter velocities and more lift providing for more lift and duration to help high RPM.  Plus they allow for higher valve spring rates for increased engine RPM's with little consequence to low RPM power and drive ability.

Lifter Specs:
body diameter .842"
lifter wheel diameter: .700"
Internals are the same as the LS7 lifter

They are more expensive than flat tappet lifters but they last longer and in some situations they can be reused if you change the cam shaft.