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 is 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 prefer a graphical interface for free, you can use the GNOME System Monitor. To start it from the desktop, go to the Main Menu Button (on the Panel) => System Tools => System Monitor or type gnome-system-monitor at a shell prompt from within X Window System. Then choose the System Monitor tab.