As memory manufacturers tried to find ways to increase the speed of SDRAMs, DDR SDRAMs were introducted. DDR stands for "double data rate".
The brain of a DDR SDRAM is similar to a regular (single data rate) SDRAM (we still have banks and rows to open). The legs of a DDR SDRAM are much faster, with faster clock speeds, longer burst cycles, and two bits per data line sent per clock cycle (hence the DDR name). That leads to very tight timing on the data lines and some features were added to help transfering reliably the data. Check the links below for more info.