Simplify the following text for a Grade 8 native English-speaking person.
Original: The main difference between DDR and DDR2 modules is that the bus on which the DDR2 memory modules are working is clocked at twice the speed of the memory cells .
Simplified: The key difference between DDR and DDR2 is that for DDR2 the memory cells are clocked at 1 quarter ( rather than half ) the rate of the bus .