A good overview of Grand Central Dispatch and Blocks. /dev/why!?!: Grand Central Dispatch
The simplistic explanation is that GCD is a combination of thread pooling and closures. The honest truth is that if thread pooling is a Motorola ROKR then GCD is an iPhone 3GS… while they are both phones and nominally do the same thing, no one would ever claim they are equal, or even i the same league. The guys at Apple who came up with it deserve a ton of credit.