Packages are applications that are packaged up to make them easier to install.
See apt
See aptitude
See dpkg
Automatically installed packages
Check if a package is installed
Determine which package a file belongs to
Extract deb file without having dpkg
Installing experimental packages
Linux Distribution Equivalents
List all packages installed on the system
List the files installed by a package
Purge removed, unpurged packages
Repair a key server error for the Google repository
Verify the files of installed packages are still exactly like they were when they got installed