Questions about XCOM: Enemy Unknown

[pacgamer]

Hi everyone,

I just got the DVD-ROM version of XCOM: Enemy Unknown. I don't play many games, so I'm curious about a few things that I'm not familiar with. (I haven't installed it yet.)

First of all the box says it requires the Steam Client. I've never used Steam before. Is this what the game uses to download updates? I wonder if it would be possible to find updated files on the hard drive so I can back them up? Also, can I play the game if the computer isn't connected to the internet?

And what about the DVD itself - do I need to keep the DVD in the drive in order to play the game? I didn't see any "No-CD" patch on the GCW page.