Debian software is grouped together in a release. All of the software in a release is available as a set of purchased or downloaded media (CDs, DVDs, or new with Debian 7, Blue-ray Discs), or as individual packages grouped in an online repository. While dpkg works only on packages already downloaded (or on media mounted locally), the other package management utilities understand offline media, and local and remote repositories, which must be configured.
All of the configuration for media or repositories resides in
/etc/apt, in a file called
sources.list and any files in
/etc/apt/sources.list.d with a
.list extension. These files can be modified manually using your preferred editor, ...