The utility free is used to examine RAM usage. Details of both free and used memory (and swap areas) are shown:
$ free total used free shared buffers cached Mem: 514736 273964 240772 0 35920 42328 -/+ buffers/cache: 195716 319020 Swap: 1794736 104096 1690640
Another useful parameter is -m, which has the effect that all sizes are expressed in megabytes:
$ free -m total used free shared buffers cached Mem: 502 267 235 0 35 41 -/+ buffers/cache: 191 311 Swap: 1752 101 1651
The really interesting information is contained in this line,
-/+ buffers/cache: 191 311
which calculates the amount of memory taken up with buffers and caches.
The parameter -d delay ensures that the display is refreshed every delay seconds. For example, free -d 1.5 produces an update every 1.5 seconds.