Memory is like short term storage for the computer. Everything that is running is stored in the memory to be used by the processor.
All of your programs use up memory. The more programs you have running at one time, the more memory it will use. However, using up memory won”t actually slow your computer down until you run out of memory.
Scenario
An old computer has 256MB of RAM. This computer is running Windows XP which uses about 200MB of RAM itself. This leaves only 56MB of free RAM. If you open MSN Messenger, Microsoft Word, and Internet Explorer these programs combined will use 157MB. This creates 357MB of RAM, being used by a computer which only has 256MB of Ram.
In this situation the computer will use the Hard drive (long term memory), as replacement for RAM. Since the hard drive runs a lot slower than RAM the computer will start to run slowly.
However, if you have 2048MB (2GB) of RAM and you only use 357MB then your computer will run fine until you use more than 2048MB of ram.