Today's Command is "free":

What it will do :

This command shows us the free and used memory in the system.

How to use it:

Lets see with an example:

$free
             total       used       free     shared    buffers     cached
Mem:       1024928     899844     125084          0      97868     558352
-/+ buffers/cache:     243624     781304
Swap:      2522164          0    2522164

The above command is showing the total amount of free & used memory(physical & swap) in the system.

Options:

-m 	To get the output in MB format.

More more information , give this command,
$man free