Newer variants of SDRAM are DDR (or DDR1), DDR2 and DDR3. Both SDRAM and DDR RAM are memory integrated circuits used in computers. SDRAM (synchronous DRAM) is a generic name for various kinds of dynamic random access memory (DRAM) that are synchronized with the clock speed that the microprocessor is optimized for.
Traditionally, dynamic random access memory (DRAM) had an asynchronous interface, which means that it responds as quickly as possible to changes in control inputs. Both SDRAM and DDR RAM have a synchronous interface, meaning that it waits for a clock signal before responding to control inputs and is therefore synchronized with the computer's system bus. This allows the memory chip to have a more complex pattern of operation than an asynchronous DRAM. This is also why the speed of SDRAM and DDR RAM is rated in MHz rather than in nanoseconds (ns).
SDRAM usually refers to first generation synchronous DRAM, which is slower than subsequent generations (DDR) because only one word of data is transmitted per clock cycle (single data rate). The second generation of synchronous DRAM memory chips was DDR (sometimes called DDR1). DDR stands for double data rate, which means the chip reads or writes two words of data per clock cycle. The DDR interface accomplishes this by reading and writing data on both the rising and falling edges of the clock signal. In addition, some minor changes to the SDR interface timing were made in hindsight, and the supply voltage was reduced from 3.3 to 2.5 V. As a result, DDR SDRAM is not backwards compatible with SDR SDRAM.
edit Clock Signal
SDRAM memory chips utilize only the rising edge of the signal to transfer data, while DDR RAM transfers data on both the rising and falling edges of the clock signal.
In a computer system, the clock signal is an oscillating frequency used to coordinate interaction between digital circuits. Simply put, it synchronizes communication. Digital circuits designed to operate on the clock signal may respond at the rising or falling edge of the signal.
The main difference between SDRAM and DDR memory is the doubled speed: DDR can transfer data at roughly twice the speed of SDRAM. PC133 SDRAM runs at 133 MHz, while 133 MHz DDR effectively runs at 133 MHz x 2 = 266 Mhz.
edit Physical difference
SDRAM has 168 pins and two notches at the connector while DDR has 184 pins and a single notch at the connector.
Typical DDR SDRAM clock rates are 133, 166 and 200 MHz (7.5, 6, and 5 ns/cycle), generally described as DDR-266, DDR-333 and DDR-400 (3.75, 3, and 2.5 ns per beat). Corresponding 184-pin DIMMS are known as PC-2100, PC-2700 and PC-3200. The numbers represent the theoretical maximum bandwidth of the DDR SDRAM in Megabytes per second (MB/s). For example, PC2100 has a theoretical maximum bandwidth of 2100 MB/s.
SDRAM was released first in 1997; DDR RAM was released in 2000.Subsequently DDR2 RAM and DDR3 RAM. DDR4 RAM is expected to be released in 2012.
edit Clock Speed
You need to check the motherboard manual to see what RAM type (whether it is SDRAM or DDR RAM) is compatible with your system before purchasing memory. The clock speed for the memory chip should be synchronous with the computer's system bus. Both SDR and DDR RAM are offered at various clock speeds; installing a version faster than a motherboard can support is a waste of money.
edit Video explaining the difference