With world+dog merrily dropping terms like FC-PGA into polite conversation, here's a layperson-friendly explanation of what they're on about:
The Flip Chip Pin Grid Array (FC-PGA) package is used on Pentium III processors, while Plastic Pin Grid Array (PPGA) packaging is used on Celerons. This does not mean Celerons have plastic pins - that would be silly.
PPGA processors have the silicon core facing down towards the motherboard. The core is covered by a heat slug helping to dissipate heat to the heatsink. But as processors become smaller (0.18 micron and below) and faster, the ability to dissipate heat from the processor core becomes more critical.
The FC-PGA package flips the silicon core over facing up, towards the heatsink. The core sits on top of the actual package and the silicon die is exposed enabling it to make direct contact with the heatsink and thus run cooler.
Simple, eh? ®