The masquerade module is designed as a tool for site designers and site administrators. It allows a user with the right permissions to switch users. While masquerading, a field is set on the $user object, and a menu item appears allowing the user to switch back. Watchdog entries are made any time a user masquerades or stops masquerading.
This module is a little helper to maintain an administrator role which has all available permissions. By default, Drupal only has one super user and this module helps improve this drastically.