With over 2400 possible GPO objects (and counting), it is difficult to know what objects exist without manually going through a GPO and looking at all of them. I recommend using Bing or Google and asking "How do it (insert issue here) in group policy?" But if you like doing more detailed research, below are some links that can help.
Group Policy Home Page: http://technet.microsoft.com/en-us/windowsserver/grouppolicy/default.aspx
Group Policy Settings Reference for Vista/2008: http://www.microsoft.com/downloads/details.aspx?FamilyID=2043b94e-66cd-4b91-9e0f-68363245c495&displaylang=en
Group Policy Settings Reference for 2003: http://www.microsoft.com/downloads/details.aspx?FamilyID=7821C32F-DA15-438D-8E48-45915CD2BC14&displaylang=en