The Drupal 7 Queues API is a few feature of Drupal and provides a first in first out data structure that is used internally by Drupal itself and can be completely customised.
The Queues API isn't just a part of the codebase of Drupal, it is used internally as part of several different processes. The Batch API, which allows lots of things to be done at once, is built upon the Queues API and provides some customised queue classes. I won't be going into the batch API in this post, but I might cover it in later posts.
The new cron system in Drupal 7 uses the Queues API heavily. It works by allowing other modules to create queue items during their normal hook_cron() calls, which are then run afterwards. Each module that wants to include an item in the system cron queue can do so by implementing a hook called hook_cron_queue_info(), which tells cron what function to callback when the queue items are retrieved.