Memory is used to store data temporarily and loses the data when power is removed. Memory access is much faster than optical, mechanical or solid state devices such as hard drives, dvd and usb pens. A typical system now should have a minimum of 512mb and high end system often have 4GB+.
There are various types of memory. The main types are:-
SDRAM – This comes in various speeds of 66,100 and 133mhz and has now been superseded by ddr. This is now in limited production and as such can be hard to source.
Rambus – Very expensive. Used in a few computers although not very popular. Must be installed in matched pairs.
DDR (Double data rate) – Available commonly in speeds of 266, 333 and 400 mhz. Only used now in cheap computers and for upgrades.
DDR2 – This has now been superceeded by DDR3 and is available in speeds of 533,667, 800 and 1600mhz
DDR3 – This is currently the most popular speed of ram
DDR4This is now starting to appear on servers and high end i series boards such as 2011 socket/>
DDR5 – This is mostly still used on graphics cards