The computer systems physical memory which is referred to as main memory or RAM (random-access memory,) which are chips capable of holding data. This is where the computer system holds current program and data that is in use. The term “memory” is somewhat ambiguous; it can refer to many different parts of the PC because there are different kinds of memory a computer uses. However, when used by itself, “memory” usually refers to the main system memory, which holds the instructions that CPU executes and the data those instructions work with.
DDR2 is a SDRAM variant that replaces DDR-SDRAM, offering greater bandwidth and density in a smaller package along with a reduction in power consumption. It also offers new features that compliment higher clock rate CPUs and 64 bit architecture.
DDR-xxx or DDR2-xxx:
PC-xxxx or PC2-xxxx