DDR vs DDR2

DDR stands for Double Data Rate and RAM stand for Random Access Memory.
DDR2 is the next generation of  DDR RAM. These both types of  SDRAM.

The key difference between DDR and DDR2 is that in DDR2 the bus is clocked at twice the speed of the memory cells, so four words of data can be transferred per memory cell cycle. Thus, without speeding up the memory cells themselves, DDR2 can effectively operate at twice the bus speed of  DDR.

Comparison chart

Voltage	2.5 Volts	1.8 Volts
Speed	200 MHz, 266 MHz, 333 MHz, 400 MHz	400 MHz, 533 MHz, 667 MHz, 800 MHz, 1066MT/s
Chipset support	All DTs, NBs, and servers	All DTs, NBs, and servers
Modules	184-pin DIMM unbuffered registered; 200-pin SODIMM; 172-pin MicroDIMM	240-pin DIMM unbuffered registered; 200-pin SODIMM; 214-pin MicroDIMM
Data Strobes	Single-ended	Single-ended or differential
Release year	2000	2003
Succeeded by	DDR2	DDR3
Prefetch Buffer	2 bits	4 bits
Package	TSOP (66 pins) (Thin Small Outline Package)	FBGA only (Fine Ball Grid Array)
Read Latency	2, 2.5, 3 Clock cycles	3 - 9 clock cycles, depending upon settings
Write Latency	1 clock cycle	Read latency minus 1 clock cycle
Internal banks	4	4 or 8