Repositories is an important part of ubuntu, through repositories we can find any program we are looking for, here are simple explanation to help us understanding what's ubuntu repositories.

What is Ubuntu Repositories ?
Repositories are literally thousands of Ubuntu programs available to meet the needs of Ubuntu users, there are two type of Repositories that is online Repositories and offline Repositories. Online repositories is repositories that provide by official, community, application developer  which can access through the Internet, we can pick (install) the package from there by adding the repository address in our package/software manager. Offline Repositories are repositories thats contain on Hard drives and DVD's, we can get thousands of Ubuntu programs on official website, download and burn into a CDs/DVDs and ever time you want to install some program, just put the CDs/DVDs.

Repositories is manage by application/software manager, through that application manager we can install, uninstall, and upgrade ubuntu program easily while also providing a high level of security, as each program available in the repositories is thoroughly tested and built specifically for each version of Ubuntu. If you have install Ubuntu you will find application called Ubuntu Software Center and Synaptics Package Manager,they are examples of application/software manager in ubuntu.

What's Mean of Universe, Multiverse in Ubuntu Repository ?
There are 4 (four) type of component of program in repositories, some component have different functions,support and purposes, they are some component of Repositories and what it's mean :

  • Main - Contain of officially supported software/application.
  • Restricted - Contain of Supported software that is not available under a completely free license.
  • Universe - Contain of Community maintained software, i.e. not officially supported software.
  • Multiverse - The Multiverse repository contains software which has been classified as non-free. This software may not be permitted in some jurisdictions. When installing each package from this Repositories, you should verify that the laws of your country permit you to use it. Also, this software may not include security updates.
On the Ubuntu CDs Installed they are contain software from the Main and Restricted components of the repositories. Once your system is made aware of the Internet-based locations for these repositories, many more software programs are available for installation. By using the software package management tools (Ubuntu Software center/Synaptics Package Manager) you can search for install and update any piece of software directly over the Internet, without the need for the CD.

How to Add and Manage ubuntu Repository ?
the way to add repository, Manage, and update program can be seen and learn from ubuntu official site bellow
That is Ubuntu repositories are, the more information that you get about ubuntu will make you understand and pround of using it.