It is important to understand how licensing used to work before Windows Server 2012, as understanding it will show how easy it is today to license your environment. In addition, if you come to manage a legacy environment one day, you will be able to verify if the licensing for this environment is correct.
One aspect of Windows Server 2008 and 2008 R2 that is different from Windows Server 2012 and 2012 R2 is that 2008 and 2008 R2 had more editions available for purchase. At that time, you were able to license the following editions:
Standard Edition
Enterprise Edition
Datacenter Edition
There are actually other editions for 2008 and 2008 R2, but they focus on specific scenarios such as Small Business Server Edition and Foundation Edition, and will not be covered here.
One important aspect of Windows Server 2008 and 2008 R2 is that these editions had differences between them, not only from the virtualization licensing perspective, but...