Views has always been a staple module for any Drupal site. It was so popular and needed that it ended up being incorporated into Drupal 8 core. So now, each new Drupal site ships with Views out of the box, fully integrated with the rest of the system and powering a great number of core features.
Essentially, Views is a tool for creating and displaying lists of data. This data can be almost anything, but we mostly use Drupal entities as they are now so robust. It provides the architecture to build and manipulate complex queries through the UI as well as many different ways of outputting the resulting data. From a module developer's point of View (yes, pun intended), much of this power has been broken down into multiple layers of building blocks, abstracted as plugins. Moreover, in keeping with tradition, there are also a multitude of hooks that are fired at different...