CanCanCan is an authorization library for Ruby and Ruby on Rails which restricts
what resources a given user is allowed to access.

All permissions can be defined in one or multiple ability files and not
duplicated across controllers, views, and database queries, keeping your
permissions logic in one place for easy maintenance and testing.

It consists of two main parts:
- Authorizations library that allows you to define the rules to access different
  objects, and provides helpers to check for those permissions.
- Rails helpers to simplify the code in Rails Controllers by performing the
  loading and checking of permissions of models automatically and reduce
  duplicated code.