This type of memory replaced the EDO SIMM standard. 168-pin DIMMS were the first standard whereby modules could be installed singly.
Compatible motherboards were fixed to one of the three bus speeds, but allowed you to use 133Mhz memory in a system with other modules of 100Mhz but obviously the 133Mhz module could only work at the limit of 100Mhz defined by the motherboards chipset.
Concerns from other vendors that Intel and the memory maker Rambus could control the memory market, lead to DDR Memory being introduced. This forced Intel to abandon exclusive support of RDRAM and lead to DDR/DDR2 support.
This is older type of memory which needs to be used in pairs of the same rated memory eg 2 x 8,16,32,64,128 modules. It can be quite expensive to purchase compared to the now more mass produce newer lines.
Unsure what type of memory you need? Use the Memory Advisor Tool [here] Or lookup the type of memory your Motherboard uses in your manual.
Online Store Links for Memory: