Like Jason says, Shock is a process not a product, worse yet it seems many brands will call any powdered chlorine product "shock", I was in the local Lowe's store yesterday and saw they had 4 different "shock" products in 1 pound bags, all the same brand, with slight variations on the name (things like shock, super shock, shock plus, etc.), 3 were powdered tri-chlor (one of which had copper algaecide added, and another had some other additive), and the 4th only available in the multi-pack box was good old Di-Chlor. Last year at Lowes the same brand only had Di-Chlor or "super strength" which was Di-Chlor plus copper (be ware of any chlorine product with copper it can cause other pool problems). By comparison last year I went into Wal-Mart and found every "shock" product they had in stock were based on Cal-Hypo.
Ike