Regarding the difference between Windows Server 2003, Windows Server 2008
and Windows Server 2012, here is a similar thread for your reference:
Difference b\w windows server 2003,2008 and 2012
Please also check the below link for more details:
What's New in Windows Server 2012
What's New in Windows Server
Hope this helps,
Feature & Roles change from version, so they add more feature in the end.
Check the feature/roles list for each version to see the differance
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