from Crucial's FAQ:
DIMMs/SIMMs:
DIMM stands for dual inline memory module, and SIMM stands for single inline memory module. The gold or tin pins on the lower edge of the front and back of a SIMM are connected, providing a single line of communication paths between the module and the system. The pins on a DIMM are not connected, providing two lines of communication paths between the module and the system, one in the front and one in the back.
EDO/SDRAM:
Extended data out (EDO) memory and synchronous dynamic random access memory (SDRAM) are two different types of memory technology. SDRAM is the newer, faster type of the two. The biggest difference between the two is that SDRAM is synchronized to the CPU clock.