We've noticed there's some confusion about the Steam version of the game versus the other versions like the one that'll be distributed through gog.com or the vault.
The short of it is that all versions all equal and all versions are compatible with one another.
However, you do need a Steam account and the game present in your Steam library if you want to take advantage of all of the Steamworks features that are integrated into the game.
Non-Steam versions can still play online or in a LAN, but won't be able to use the Steamworks features. In that case the Steamworks part of the game will become grayed out.
Steamworks is being used for online lobbies, leadership boards and inviting friends. These features are not available if you don't have the game installed on Steam. What you can do with non-Steam versions is play directly with a friend over LAN or online if you know each others IP.
All versions are DRM free btw (including the Steam version) but feature a secret surprise for pirates.
Hope that clears it up.
Post updated on August 7th 12:04 CET to clarify Steamworks features
Last edited by Lar_q; 07/08/13 10:06 AM.