On modern computers RAM usually comes in the form of Dynamic Random Access Memory or DRAM. DRAM is RAM that has to be regularly refreshed by the memory controller. DRAM is cheaper than the alternative memory called Static RAM but is also slower than SRAM.
Generally speaking, the more RAM you have in your PC the faster your PC operates. RAM is used to store running programs and associated data. It is also used to cache access to slower storage mediums like hard disk drives.
There are different standards for DRAM. As the technology evolves older standards are replaced with newer and faster ones. Some of the standards of DRAM are:
SDRAM
DDR
DDR2
DDR3
RAM comes in the form of a module (pictured on the left) referred to as a Single Inline Memory Module (SIMM) or nowadays Dual Inline Memory Module (DIMM). DIMMs are inserted into a socket on the motherboard.