Microsoft has said that it is working on purchasable Xbox 360 games for users on Xbox One.
While at the moment you can play 360 games you currently own or get free from Games with Gold, people have been complaining that there's no way of actually buying Xbox 360 titles from the Xbox One. Thankfully then Microsoft is working on a solution to this.
In the meantime, there is a way around the issue, either by purchasing the Xbox 360 game of your choice from a store (if you can find it) or you can go to Xbox.com and buy them there, as the new title will then appear on your "Ready To Install" list on Xbox One.
Remember that not all games are currently backwards compatible, so make sure you do your research if you want to play an Xbox 360 game on Xbox One.