Can any tell me, how to write a research report describing how Groups and Organisational Units within the Windows Server 2008 operating system.
Show me the area I can info for my research

Simply put;
Groups - security objects for assigning permissions, normally to folder and server access.
Organisational Units - a container of stuff, normally users and computers to which a set of group policies can be applied.

You really need to go back to server 2000 for more indepth information as that is when they where introduced..