Memory Usage

The free command displays the total amount of physical memory and swap space for the system as well as the amount of memory that are used, free, shared, in kernel buffers, and cached.

             total       used       free     shared    buffers     cached
Mem:        256812     240668      16144     105176      50520      81848
-/+ buffers/cache:     108300     148512
Swap:       265032        780     264252

The command free -m shows the same information in megabytes, which are easier to read.
             total       used       free     shared    buffers     cached
Mem:           250        235         15        102         49         79
-/+ buffers/cache:        105        145
Swap:          258          0        258

If you would like to use a graphical interface with free, you can use the GNOME System Monitor. To start it on the GNOME desktop, go to the Main Menu Button (on the Panel) => Programs => System => System Monitor or type gtop at a shell prompt from within any X Window System desktop. Then choose the Memory Usage tab.

Figure 20-2. GNOME System Monitor