Backwards-compatible with the entirety of the 5th edition of the game, the new books are a big new step forward for the game, reflecting a decade of iteration and evolution. For new players this will be the definitive manual to own for D&D 5E, and for existing players this will bring loads of playtest material, erata, and "common house rules" into the fold of common rules. 

For existing fans, this Player's Handbook will introduce lots of the changes to classes that have been tested out via Unearthed Arcana, such as the overhaul to the Druids’ Wildshape ability and complete retooling of Monks. 

The revisions were first announced in 2022, under the umbrella brand of One D&D. While that name has been dropped and the new versions are just called "Player's Handbook, Dungeon Master's Guide and Monster Manual", we've opted to call them "Revised" editions in order to help distinguish them in web searches. They’re still the fifth edition rules, however, the changes are substantial enough to warrant a reissue of the PHB we've been playing with since 2014. All existing content will be compatible with these books - so you can still play with your Deck of Many Things and Volo's Guide - nothing about the baseline rules is changed.