Issue the command free:
total used free shared buffers cached
Mem: 1035296 864012 171284 0 39328 525800
-/+ buffers/cache: 298884 736412
Swap: 2008116 20044 1988072
The highlighted value on your system is the amount of free memory. Linux keeps information in memory (cached) so that it doesn't have to go to disk. This speeds up the operation of the system.