http://www.youtube.com/watch?v=nyYW9AOWh-8
QuoteGranted, if you want to play online, you will need an Internet connection obviously, and if you want to show off achievements you will need a Galaxy account. But online will be "completely optional – it is up to the player to decide which features they want to use."
The client will provide the option for automatic game updates and the ability to stay in-touch with friends.
It also won't matter which digital store you purchased your game on, so you will not need any third party apps or accounts – think Cross-Play and you have it.
So, if you want to play with your buddy on Steam, you can. Awesome.