RAM (Random Access Memory) is device which belongs to system memory category. RAM (Random Access Memory) allows data to be stored on cache which can be used randomly in a faster manner.
In general words it provides space to store information before writing or reading it from hard drive and also help different programs to run in better way. More memory cache means faster your pc can respond. These RAM (Random Access Memory) also been through lot of changes and innovations but the vast change happened in the last 5 year only.
It is the key feature to run all programs and as well as for operating system. No operating system can run without RAM (Random Access Memory). Selection of RAM (Random Access Memory) also depends on the operating system on which a person wants to use it.
Like for Windows XP RAM (Random Access Memory) of 512 MB will be sufficient and one can increase according to his/her usage but for latest operating systems like Windows Vista and Windows 7 there is minimum requirement of 1 GB of RAM (Random Access Memory).
RAM (Random Access Memory) is now days available in three forms DDR, DDR2 and DDR3. One should check the compatibility with his operating system and motherboard while buying it.
DDR RAM was introduced in year 2002 to replace the ancient SDRAM. This DDR RAM was faster with 184 pins. It is used for speed of PC2100, PC2700, PC3200 (DDR400), and PC4000 (DDR500).
Then with in few years DDR 2 came into fashion and become need of every computer, the main reason was more pins that are DDR 2 had 240 pins which made work faster and efficient. It supports speed of PC2-3200 (DDR2400), PC2-4200 (DDR2533), and PC2-5300 (DDR-667).
But they are replaced with DDR3 which came into fashion from 2008 which are faster and effective then DDR2.