The autoload module is a utility module. It allows other modules to leverage PHP 5's class autoloading capabilities in a unified fashion. It does, naturally, require PHP 5.1.2 or later.
You do not need this module unless you are developing a module that makes use of classes or you are installing a module that depends on this one.
The 1.x branch uses a manual info-hook. The 2.x branch includes a backport of the Drupal 7 self-learning registry as well as the info hook for backward compatibility.
Because this functionality is already in Drupal 7 core, there will never be (or need to be) a Drupal 7 version of this module. Leveraging this module for Drupal 6 class-using modules, however, will make moving to Drupal 7 easier.
See the included README.txt file for information on how to use this module in your own modules.
NOTE: Remember to clear all caches in your module's hook_enable() if your module relies on autoload. Autoload information is cached and your class may not be found if the autoload cache is not cleared.