sources.list file in Ubuntu

This is something I got to know recently while trying to install a new software package on Ubuntu. 

In Ubuntu, we use apt-get command line tool very often. It is, Ubuntu's Advanced Packaging Tool (apt) which performs functions such as installation of new software packages, upgrade of existing software packages, updating of the package list index, and even upgrading the entire Ubuntu system. 

sources.list file located in /etc/apt folder, is the default file which contains the details about the repositories needed when installing or upgrading the applications. When the system needs to know from where it may download the related stuff to install/upgrade it basically goes through this file.  

Repositories are servers which contain sets of packages. Ubuntu uses apt for package management. apt stores a list of repositories or software channels in this file. 

We can add a new repositories by simply adding a new line to this file. You can learn about the format of the entries in this file here. 

Once the editing stuff done, we have to do one more thing. Just to make apt aware of the changes done to sources.list, run;

sudo apt-get update

You can read further in Ubuntu help page.