RAM or Random Accesses Memory can be compared to a warehouse. Information is moved from the Hard Drive and on  to it when you Start up a program. If It can the computer will load the whole program into RAM.  The reason for this is that ram Is much faster than the hard drive a usually can keep ahead of the CPU. This helps your Programs run smoother with out any Pauses. Most of the times newer programs are just too big for the amount of ram you have. This is why you want as much ram as possible.