RAM, short for Random Access Memory is the short storage area for your PC. Often just called memory or system memory RAM is just an electromagnetic storage that loses all its data once the power has been removed. RAM is used by your operating system and other programs and games in order to store data that is required at speed. Computers with large amount of memory often perform faster simply because the CPU is not idle waiting for data to be retrieved from slower components such as the hard drive and can be stored in memory.
Why is RAM considered Random Access
RAM can be accessed by the computer for any piece of data in any cell of the memory bank. There is no need to go through the entire memory bank to get to the data you require. The opposite to Random access is serial access. SAM or Serial Access Memory needs to be read from the start to get to the data you require. An example of SAM storage would be a cassette tape. Of course there are things which fit in-between these two examples such as a CD or DVD. Because these have tracks they can be random access to a point but then need Serial access to get to the exact spot required.
How does RAM store data
RAM consists of many capacitors and transistors. A capacitor and a transistor are paired together to make a memory cell. The capacitor represents one "bit" of data, the transistor is able to change the state of the capacitor to either a 0 or a 1. the Zero's and ones when read in a sequence represent the code which the computer understands. This is called binary data because there is only two states that the capacitor can be in.
In order for a capacitor to achieve a value of 1 it needs to be filled with electrons. To achieve a value of 0 the capacitor needs to be emptied of electrons. You may of noticed when purchasing RAM that many types of RAM you buy are called DRAM or SDRAM. DRAM or Dynamic Random Access Memory has a small problem with the capacitors holding a value of 1. It is called dynamic RAM because its state of 1 or 0 needs to be constantly refreshed on order to stay in the correct state. In order to demonstrate this and for a much more detailed look into the workings of RAM a good site site is How stuff works. This article will show you a much more technical look at the workings of RAM and has a great Diagram to illustrate the nature of Dynamic Random Access Memory
There is a type of RAM that doesn't have to be refreshed constantly. This is called SRAM or Static RAM. Static RAM uses a type of Flip Flop to hold the data in the cell. This can take around 6 transistors on a chip per cell rather than just the one., The consequences are that Static RAM needs more chips per Mb than DRAM and therefore is much more expensive.
Types of RAM
Down the years the face of RAM has changed dramatically in the early days of computing we had the SIMM. SIMM was a Single Inline Memory Module. Moving on the next logical step was the DIMM, Double Inline Memory Module. The introduction of DIMM's brought with it new speeds and sizes to give computers more power than ever before. The big advances in recent years has been the introduction of DDR RAM (Double Data Rate). DDR effectively doubled the speed that RAM could transfer data without actually increasing the MHz. E.g. a stick of PC133 (133MHz) RAM with DDR would effectively transmit data at 266MHz but still have a bus speed of 133Mhz.
The other big memory type of the recent past is RAMBUS memory. Rambus memory uses a memory module called a RIMM (RAMBUS Inline Memory Module) RIMM's are specials high speed memory chips working upto 800Mhz. They require special motherboard support and are priced higher than that of standard memory modules.
Measuring the Speed of RAM
Like CPU's RAM is measured in MHz. The higher the MHz the greater the speed of the RAM. To keep this calculation simple when dealing with DDR RAM, retails often simply put the effective MHz rating. For example a 133Mhz DDR module would be advertised as 266Mhz. That is basic speed test of the memory module. However there is another important factor in high performance memory and that is CAS latency.
CAS meaning Column Access Strobe, the basics of CAS latency are that CAS latency is the amount of clock cycles it takes for the response from the memory from a query. Shorter is obviously better. CAS-2 is a 2 cycle delay and CAS-3 a 3 cycle delay. To learn more about CAS latency Corsair have a good article here