PC SDRAM is a loose general term for SDRAM that runs at 66 MHz and has an SPD chip for compatibility with P-II motherboards.

PC100 SDRAM refers to PC100 SDRAM chips or DIMMs that meet INTL PC100 qualification standard. These parts are designed to run at 100 Mhz front side bus (FSB) speeds.

Registered SDRAM - This is SDRAM module with Register for Address and Control Signals. Registered DIMMs reduce the loading of DIMM to the motherboard so that larger capacity DIMM modules and more DIMMs can be populated on a motherboard. It is a technique used widely on servers to increae the amount of memory the system can support. The Registered DIMM is a little slower in access timing versus that of the unbuffered counterpart.