DDR – Double Data Rate memory. Originally data was transferred from memory to the CPU every clock signal. The more DRAM capacity and the higher the speed the better, as server and PC CPUs can get through more work with less waiting for memory contents to be read or written. As a way of increasing the rate DDR transfers data at the start of the clock signal and also at its end, doubling the data rate per clock signal.
There are five generations of DDR technology, as at November 2022, with a sixth generation in prospect. Standards are set by JEDEC and data rates can be exceeded