DDR2 is the second generation of DDR modules.  It applies the same 64-bit wide data bus and also supports dual channel memory technology but it is faster and uses less power than DDR1.  New advanced features were added at the chip level which provides for enhanced speeds and better signal integrity.

DDR2 is not backwards compatible with DDR1.  Modules come in 240 pins versus the 184 pins of a DDR1.  They have the same length (5.25 in) but the notch is placed in a different position to prevent it from being accidentally plugged into a different socket.  Before upgrading your system, check the specifications of your motherboard for compatible memory support.