DDR2 introduces some new features which allow it to ramp up to much higher speeds (with correspondingly higher bandwidth) and higher memory densities, all the while using less power. DDR2 memory uses a new form factor, a 240 pin DIMM (Dual Inline Memory Module) which is *not* compatible with current DDR memory slots. Upcoming chipsets by Intel and other manufacturers will support DDR2 specifically, and are not backwards compatible.
DDR SDRAM for desktop computers DIMMs have 184 pins (as opposed to 168 pins on SDRAM, or 240 pins on DDR2 SDRAM), and can be differentiated from SDRAM DIMMs by the number of notches (DDR SDRAM has one, SDRAM has two). DDR for notebook computers SO-DIMMs have 200 pins which is the same number of pins as DDR2 SO-DIMMs. These two specifications are notched very similarly and care must be taken during insertion when you are unsure of a correct match