Games With Gold is Microsoft's loyalty program for Xbox Live Gold subscribers. As part of it, subscribers get free games (on the Xbox One and the Xbox 360) every month.
Xbox One backward compatibility is currently available only to Xbox Preview Program members, but will launch for everyone in November, Microsoft also announced today.
There will be more than 100 games available at launch, with "hundreds" more coming later.
You can see the initial lineup of supported games here.
The Xbox One outsold the PlayStation 4 during the week of E3, something that Microsoft thinks is at least partially attributable to backward compatibility.