Random Access Memory
Random Access Memory (RAM for short) is the work area used to hold both instructions and data whilst the computer is working on it. It is Random Access in that it takes the computer the same amount of time to retrieve data from any part of the memory, unlike a hard drive -for example- where the position of the data on the disc will affect the speed of it's retrieval.
RAM is said to be volatile in the sense that anything in RAM is lost when the computer is switched off, as opposed to data on drives which, once written, is retained even after power down.
Random Access Memory .. is the work area used to hold both instructions and data whilst the computer is working on it
So, why do we use RAM? The short answer is because it's fast and (relatively) cheap! Reading and writing data from a hard drive (-or even an SSD) is slow, so any computer that dispensed with RAM would be too slow to be of much use. As a result, data is normally read from a drive, into RAM, where the processor works on it. If the output is important, it can then be written back from RAM to a drive to persist it, after the power is pulled.
There are many different types of RAM available, but the overwhelming majority of memory available for home PCs currently falls into two main types:
DDR2 (clock doubled): this is the older type of memory. It is generally cheaper but is gradually being phased out in favour of DDR3, so if you are buying a new Motherboard, go with DDR3 if you want to keep your options open for future expansion. I would advise only using DDR2 if you will be using a existing motherboard which requires it
DDR3 (clock trebled): DDR3 is the latest incarnation of memory chips. It is generally available in quicker speeds but you do have to pay more for it. The majority of new Motherboards are now switching to DDR3
Within both DDR2 and DDR3, you will see a number of different speeds (-specified in megahertz or MHz): the higher the MHz value, the faster the RAM. So, for example, both DDR2 and DDR3 are available running at speeds of 1066MHz, 1333MHz, 1600MHz. In this example, 1600MHz is the fastest - and also the most expensive!
You should also pay attention to the latency values of the RAM: this is the delay in accessing RAM on the DIMM -and the lower the latency, the better the memory will perform.
There are a number of things that you need to verify before you order your RAM, notably:
If you are adding RAM to a laptop, you are likely to need the smaller SO-DIMMs ..
If you are adding RAM to a laptop, you are likely to need the smaller, SO-DIMMs whereas if you are adding it to a desktop/server, you almost certainly need 240-pin DIMMs. SO-DIMMs are incompatible with DIMMs - so make sure you order the correct type! Also, check the number of pins on your motherboard (-the norm is 240 pins for most DIMMs currently, but older motherboards can have differing numbers)
Check your Motherboard supports the RAM. The best way to do this is to go to the motherboard manufacturers website and check the list of supported RAM for that board
If you are adding more than one DIMM/SO-DIMM, try to keep them identical if possible, to avoid incompatibility problems. If not identical, try and keep them as similar (size, clock speed, latency)
Where possible, check the feedback from other users on your desired RAM: most sites allow users to review products and leave feedback, so be sure to take note of the experience of others