APT HOWTO
Chapter 3 - Managing packages

3.1 Updating the list of available packages

The packaging system uses a private database to keep track of which packages are installed, which are not installed and which are available for installation. The apt-get program uses this database to find out how to install packages requested by the user and to find out which additional packages are needed in order for a selected package to work properly.

To update this list, you would use the command apt-get update. This command looks for the package lists in the archives found in /etc/apt/sources.list; see The /etc/apt/sources.list file, Section 2.1 for more information about this file.

It's a good idea to run this command regularly to keep yourself and your system informed about possible package updates, particularly security updates.