Hi Dreadking

For more information go to the Steam website here:

General stuff and an idea of what the app looks like

http://store.steampowered.com/about/

and here

For more frequently asked/answered questions and known issues

https://support.steampowered.com/

As an immediate response. You do not have to have steam connected to the internet while playing the game in single player mode. You can chose to play offline. A one time connection to the Steam server for authentication is all that is required to play the game.