A DDR3 DIMM (Double Data Rate 3) is a memory card. DDR3 is memory, and DIMM is the name for the part installed. This card replaced the DDR2. It is faster and allows information to be transferred at a higher rate of speed. It contains 240 pins and one off-set notch (which insures it is not mistaken for others). This notch only allows it to be used in certain motherboards. Other memory cards, like the DDR2 or the DDR (now relatively outdated), have notches in different places so that they cannot be interchanged.