What is the difference between Revisions and Service packs ?
Service packs contains bug fixes, feature additions etc to an OS or Application.
Ex: Windows 2008 Sp1
Revisions are major changes done at the Kernel level. Revisions also include Service packs that are previously published.
Ex: Windows 2008 R2, SQL Server 2012 R2
So in general, Revision is the top and latest version to a service pack.
Windows 2008 R2 is the latest version compared to Windows 2008 SP2