Group Policy has been with us for well over 12 years now and has turned out to be a good tool for deploying configurations to your users, servers and clients. A summary of Group Policy in general is beyond what I want to say here so for anyone looking for that before reading on have a look here.
A major tenet of Group Policy and Active Directory site, domain and OU design is to group users by common denominators and configure Group Policy for them. For instance you might want to use a geographical approach and group your users according to geograpihal location. All users from Europe in one OU and all from Asia in another. Each would get a GPO setting the common configuration that all users in any give location should have. Another approach would be to group by function. Lets say we place all users beloning to our R&D deparment in one OU, regardles of physical location.