The purge option

The purge option, when used, not only removes a package, but also removes a package configuration file. This is ideal because most of the time when we uninstall a package using uninstall, it leaves unwanted configuration files in our system. Here is how we use the purge option: 

root@ubuntu:/tmp# apt-get purge virtaal
Reading package lists... Done
Building dependency tree      
Reading state information... Done
The following packages will be REMOVED:
  virtaal*
0 upgraded, 0 newly installed, 1 to remove and 5 not upgraded.
After this operation, 0 B of additional disk space will be used.
Do you want to continue? [Y/n] y
Removing virtaal (0.7.1-1) ...
Purging configuration files for virtaal (0.7.1-1) ...
root@ubuntu:/tmp#

Great!

Whenever we install a package using the apt utility, it uses repositories to download the packages into the cache. By default, when we install a Debian distribution, the installation comes with official repositories. These are stored in the /etc/apt/sources.list file. Let's take a look at that file:

root@ubuntu:/tmp# cat /etc/apt/sources.list
#deb cdrom:[Ubuntu 16.04 LTS _Xenial Xerus_ - Release amd64 (20160420.1)]/ xenial main restricted
 # See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://us.archive.ubuntu.com/ubuntu/ xenial main restricted
# deb-src http://us.archive.ubuntu.com/ubuntu/ xenial main restricted
 ## Major bug fix updates produced after the final release of the
## distribution.
deb http://us.archive.ubuntu.com/ubuntu/ xenial-updates main restricted
# deb-src http://us.archive.ubuntu.com/ubuntu/ xenial-updates main restricted
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## universe WILL NOT receive any review or updates from the Ubuntu security
## team.
deb http://us.archive.ubuntu.com/ubuntu/ xenial universe
# deb-src http://us.archive.ubuntu.com/ubuntu/ xenial universe
# deb-src http://security.ubuntu.com/ubuntu xenial-security universe
deb http://security.ubuntu.com/ubuntu xenial-security multiverse
# deb-src http://security.ubuntu.com/ubuntu xenial-security multiverse
root@ubuntu:/tmp#

The entries that start with deb refer to where to search for packages. The entries that start with deb-src refer to source packages.