This script removes package file lists. APT stores a copy of those files (in "/var/lib/apt/lists/") and searches are done within those local files to avoid going through the network for every search. An "apt-get update" will repopulate the necessary information and recreate the list file. The list files should be recreated as they get lost over time.
This script runs the "clean" command. It clears out the local repository of retrieved package files, meaning that it deletes downloaded packages (".deb") already installed (and no longer needed). It clears the whole "/var/cache/apt/archives/" directory that contains a copy of each downloaded ".deb" file. It clears the cache but does not delete the lists.
This script runs the "autoclean" command which removes package files that can no longer be downloaded, and are largely useless. This allows a cache to be maintained over a long period without it growing out of control.
This script runs the "autoremove" command, which removes packages that were automatically installed to satisfy dependencies for other packages and are now no longer needed. This script also purges removed packages, which means that the configuration files of the removed packages are deleted too.