DDR - One memory operation on the rise of a cycle, another at the fall (thus, double data rate)
DDR2 - Same as DDR, with an internal clock runing at half the external clock. This allows it to handle twice the frequency, thus being up to twice as fast (the lower frequency on the internal clock causes it to have a bigger latency, so typically a DDR2 memory doesn't run twice as fast as DDR 1 when it doubles its frequency.
DDR3 - Basically the same trick again, doubling DDR2 peak bus frequency, and further increasing latency.
Also, every iteration of memory technology lowers operating voltage, thus reducing heat and power consumption.