The file civinode_utils.inc makes up the CiviNode Utility Library.
The core concept is to create a layer on top of the CiviCRM public APIs that is easier to use in Drupal, and takes care of the more difficult and error prone parts of that API. It takes care of fetching meta data about contacts and profiles, handles basic operations on CiviCRM groups, implements basic Drupal UI widgets like group and profile selectors, and handles access to the main look up tables. It also implements a simple access control list system (ACL) that works with Drupal's node access system.
I'm also using the library as a place to concentrating testing and "best practice": if enough of us use the same wrappers, it's a lot more likely that the key "paths" through the code will get thorough testing, and will work for more modules.
The code is extensively commented, and is almost "API Module" friendly. We'll put up some of these docs shortly.