FCPGA, Flip Chip Pin Grid Array, This is the most common Intel processor socket nowadays.
PPGA, Plastic Pin Grid Array, The socket type before the FCPGA. Used by Celeron I. Celeron II have FCPGA.
You need an adapter to use a PPGA processor with a FCPGA motherboard and vice versa. If you're shopping for a new mobo, get a FCPGA as they can host both PIII and CeleronII without the need for adapters.