What are the packages list in Ubuntu?

I've seen several times people talking about apt-get and packages list, I know what is apt-get and what it does, but what are the packages lists?

The package list, as it names indicates, is the list of all packages contained in all the repositories in your sources.list configuration files. Most of the package managers (including APT) uses packages list.

This lists reside in the /var/lib/apt/lists/ directory and apt systems update them whenever you update the lists (please, note that there is a difference between update and upgrade) apt will fetch the lists from the repositories and store it locally for future uses. Whereas you run any command in apt this list are loaded an processed and your commands (like install, remove, upgrade).

Reference:

• https://www.debian.org/doc/manuals/debian-reference/ch02.en.html