Because RAM is the amount of physical memory that an operating system will use and utilize. If you are unsure about the amount of physical memory windows is using then there are ways of finding out how much RAM you have installed into your computer. Different computers are better at spreading out the amount of RAM used. Operating systems like Windows are more efficient in the amount of RAM they use, whereas Linux and Ubuntu are rubbish. When asking in a shop listen out for 4GB(Giga-Bytes) and DDR3 or above etc. These systems will be better but inevitably more expensive. Also ensure you get a good processor. (Dual Core or i3 etc.)
