APT and dpkg

1. apt(8)

   Command-line front-end for installing, upgrading and removing packages.

2. apt-get(8)

   Scripted package operations: install, dist-upgrade, autoremove.

3. apt-cache(8)

   Querying package metadata with search, show and policy.

4. sources.list(5)

   One-line and deb822 repository source formats.

5. apt_preferences(5)

   Pinning, priorities and version selection.

6. apt.conf(5)

   APT configuration directives and conf.d files.

7. apt-secure(8)

   Repository signature verification.

8. dpkg(1)

   Installing, removing and querying .deb files at a low level.

Each section links to the official page. Titles and summaries by Extra Credit; the docs belong to their authors.