DDR2 Memory is an upgraded version of the DDR or DDR1 Computer Memory. DDR2 Memory stands for Double Data Rate Memory and is the second version of the DDR Memory. DDR2 is also an upgraded type of SDRAM, which allows the computer's memory to run faster than the conventional computer memory.
Although both of the DDR1 and DDR2 Memory are SDRAM, the two types of memory are not interchangeable when it comes to the computer's motherboard. Their interface setup is different, the chipset and memory setup are in a particular arrangement.
Essentially, the downfall to this is that the DDR2 Memory is not compatible with the standard DDR Memory motherboard slots. So your computer will have to support DDR2 Memory for you to upgrade your systems memory.
To differentiate the two types of memory, each type has a different pin count and layout for their unique memory modules. DDR2 Memory uses a new form factor, a 240 pin DIMM (Dual In-line Memory Module) layout. Many computers that were available after 2003 were only set-up to work with the new DDR2 Memory layout and were not backwards compatible.
A benefit of DDR2 Memory is it runs in half the time of the DDR internal clock rate. DDR2 Memory is faster and more efficient than its predecessor. The speeds of DDR2 Memory range from 400MHz to 800MHz.