The User Profile Service in SharePoint is responsible for synchronising user data to and from SharePoint to identity stores such as Active Directory, LDAP and many others.
Questions pertaining to the setup and provisioning of the User Profile Service Application, synchronization of user profile data, the Forefront Identity Manager components in SharePoint and possibly others should use this tag.
The User Profile Service Application (UPA) is the component responsible for keeping SharePoint user's information up to date. It can read data from any source, and can be configured to write properties back to the originating identity store. This allows users to update their own SharePoint profiles, and have their data written back to for instance their Active Directory account, which in turn would update data in the Exchange Server address lists etc.
In SharePoint 2010 the UPA has been the subject of much debate, as the successful setup and provisioning is riddled challenges. Only by performing a very particular set of tasks can one hope for success in making it run smoothly. Spencer Harbar is a SharePoint MVP and MCM who has written a very detailed blog post on making the UPS work.