While Visual Studio Online continues to provide the fastest delivery of new ALM / DevOps features, the majority of users still make use of the on-premises Team Foundation Server (for now at least).
Today Microsoft released the next major on-premises version, Team Foundation Server 2015. With any new major version comes a set of new requirements and compatibility items to consider when planning your upgrade or new adoption. There is some good, though lengthy, documentation on that here which I’ve summarized for easier reference during your planning. Download
Good luck with the upgrades… and remember the TfsPreUpgrade tool if you have a large system.
Mike