As noted earlier, Drupal provides several core modules. These modules cannot be
disabled, as their capabilities are integral to the standard functioning of Drupal. Just
like other modules (including the ones we will be writing), core modules function by
implementing Drupal hooks.
As Drupal invokes these hooks, each core module will respond as necessary,
performing crucial functions at specific times during the course of a request.
we will discuss several core modules in more detail, explaining what purposes these modules serve.