--Central repository in Windows Servers, to manage users roles, rights, access & policies of each individual pc on an organization.
--Plus point of sharePoint is that it can also integrate with Window Base Authentication, which depend on Active directory.
--In active directory environment, sys admin can easily make policies for each individuals or create groups of users for rights,access on the environment in organization.
--You need to configure AC in you SharePoint Server
--Then few steps to integrate with SharePoint.
--This is all what I thought at a galance on your question, you may google and found more info plus how to integrate with SP 2010.