Adding Repository(Software source) in UBUNTU / DEBIAN LINUX

# cat /etc/*release* | grep DISTRIB_CODENAME
DISTRIB_CODENAME=raring

Back up the Old Repolist file
# cp /etc/apt/sources.list /etc/apt/sources.list.backup

Repository List file /etc/apt/sources.list .Here is the format to read the content of this file

deb: These repositories contain binaries or precompiled packages. These repositories are required for most users.
deb-src: These repositories contain the source code of the packages. Useful for developers.
http://us.archive.ubuntu.com/ubuntu: The URI (Uniform Resource Identifier), in this case a location on the internet/ or a local server URL.
raring: is the release name or version of your distribution.
main , restricted & multiverse : are the section names or components. There can be several section names, separated by spaces.

New Entry is
deb http://us.archive.ubuntu.com/ubuntu raring main restricted multiverse

Just visit this uri in Webrowser http://us.archive.ubuntu.com/ubuntu to see the multiple folder inside ,which are the components.

Refresh the package list and install softwares.
# apt-get update
# apt-get install gedit