SDRAM
Alternatively referred to as SDR-RAM, SDRAM is short for Synchronous DRAM and is a type of DIMM memory that synchronizes itself with the computer's system clock to provide a synchronization between the memory and the computer processor. Being synchronized allows the memory to run at higher speeds than previous memory types and asynchronous DRAM and also supports up to 133MHz system bus cycling. Since 1993, this has been the prevalent type of memory used in computers around the world. In the below picture is an example of a SDRAM DIMM. The original type, named SDRAM, up to the current type, DDR3, are all derivatives of the SDRAM memory type.
Also see: DIMM, DRAM, Memory definitions, RAM