Update: Your physical copies of Xbox 360 games will work too. Apparently the Xbox One will download the digital version of the game and the disc will be used for verification purposes.
Original article: After a stunning presentation of Halo 5: Guardians gameplay footage and the reveal of Recore, a new Xbox One exclusive from Keiji Inafune and the makers of Metroid Prime, Phil Spencer, the head of Microsoft’s Xbox division stunned those attending Microsoft’s E3 press conference by announcing that Xbox One will be backwards compatible with Xbox 360 digital titles.
Your digital Xbox 360 games will show up in your Xbox One game library at no extra cost to you. Unfortunately, the entire library of Xbox 360 games that will be available is limited to about 100 games but Microsoft will work on adding more Xbox 360 titles to Xbox One.
The backwards compatibility feature was demonstrated by Mike Ybarra playing Mass Effect on Xbox One. Core system features such as screenshots, broadcasting and multiplayer will be left intact.
Backwards compatibility should begin no later than this summer with the first 100 titles playable during the holiday season. Xbox Live Preview members can begin playing the initial group of Xbox 360 titles today.