RAM

Random Access Memory (RAM) provides space for your computer to read and write data to be accessed by the CPU (central processing unit). When people refer to a computer's memory, they usually mean its RAM.
If you add more RAM to your computer, you reduce the number of times your CPU must read data from your hard disk. This usually allows your computer to work considerably faster, as RAM is many times faster than a hard disk.
RAM is volatile, so data stored in RAM stays there only as long as your computer is running. As soon as you turn the computer off, the data stored in RAM disappears.
When you turn your computer on again, your computer's boot firmware (called BIOS on a PC) uses instructions stored semi-permanently in ROM chips to read your operating system and related files from the disk and load them back into RAM. SDR, DDR, DDR2, and DDR3 RAM
There are several types of RAM used in modern computers. Prior to 2002, most computers used single data rate (SDR) RAM. Most computers made since use either double data rate (DDR), DDR2, or DDR3 RAM. DDR2 is able to achieve faster transfer rates to prevent limitation of your CPU's performance, and DDR3 technology takes these advancements even further.
Note that these RAM technologies are not interchangeable. One type of RAM will not function if installed with another type, and physical differences in the RAM modules prevent them from even being inserted in the same computer.