Repositories There are thousands of programs available to install on Ubuntu. These programs are stored in software archives (repositories) and are available for installation over the Internet. This makes it very easy to install new programs. It is also very secure, because each program you install is thoroughly tested and built specifically for Ubuntu.
Sponsored Link
Some times you might see the following error message when you try to install any package using apt-get or synaptic
E: The package webmin needs to be reinstalled, but I can't find an archive for it.
E: The package mfc8500lpr needs to be reinstalled, but I can't find an archive for it.
Some time you might find some applications are having only .rpm files but you want a .deb package for your debian,Ubuntu and other debian derived ditributions.If you can't find .deb debian package in any of the debian,ubuntu repositories or elsewhere, you can use the alien package converter to install the .rpm file.
APTonCD is a tool with a graphical interface which allows you to create one or more CDs or DVDs (you choose the type of media) with all of the packages you've downloaded via APT-GET or APTITUDE, creating a removable repository that you can use on other computers.One thing you need to remember this will create backup all the packages installed using apt-get,synaptic because these package arcives are stored in /var/cache/apt/archives