A 2004 article from SearchWindowsServer.com but still applies. by James Michael Stewart, Contributor

One or more group policies (GPOs) can be defined for each domain, site, and organizational unit (OUs) in a forest (that is, an entire Active Directory domain network). All of the GPOs can be managed from a Windows 2000 or Server 2003 domain controller system through the Active Directory Users and Computers utility as well as several others.

Some settings are not explained in the Microsoft spreadsheets. However, there is an explanation of many of them, includng the security settings, here: http://technet.microsoft.com/en-us/library/cc785710(WS.10).aspx

See particularly the entries under "local policy".

Wonder what all those GPO settings mean? Microsoft has provided some guidance. They have produced spreadsheets that identify all of the settings for the .adms included with Win2k3, plus addendums for Win2k8 and Win2k8R2. They include short text explaining what the settings do.

For convenience, I've attached them. They came from:

Group Policy Settings References for Windows and Windows Server
(also referred to as "Group Policy XLS")
http://go.microsoft.com/fwlink/?LinkId=54020