What does the command do?

You can use the whatis command to get a brief description of what a command does. For example, if you want to know the purpose of the free command, you can run the whatis free command:

elliot@ubuntu-linux:~$ whatis free
free (1)             - Display amount of free and used memory in the system

As you can see, the free command, as we already know, displays the amount of free and used memory in the system. Cool! Now let's see what the df command does:

elliot@ubuntu-linux:~$ whatis df
df (1)               - report file system disk space usage

Finally, let's see what the which command does:

elliot@ubuntu-linux:~$ whatis which 
which (1)            - locate a command

As we already know, which displays a command's location.