The DRM consists of the game being locked to your personal Steam-account. The game cannot be installed without; using an existing, or creating a new Steam Account.
Before you buy, note that Infinity Ward removed: Lean-function, Dedicated Servers, 18+ people server options etc. So if you're buying it for the multiplayer experience, prepare to be dissapointed - COD4 and COD WAW are still better when it comes to multiplayer IMHO.
The multiplayer in COD MW2 is now P2P-based which results in poor latency for everyone except the host. For above average latency you'll need to completely open you NAT, which leaves your internet connection vunerable to exploits. You cannot have a list of favourite servers. All in all it's complete and utter sh*¤
Exactly how does steam work, or in other words, will it open up my computer to exploits such as SecuRom does?
Nope. All your games are tied to your account. What's great about this is, if you ever have to replace your PC, you just reinstall Steam and download all your games. It doesn't matter that you have a completely new machine because it doesn't tie your games to the machine itself.
That's about it. Steam itself is the DRM and doesn't use any other DRM. Now, if the game you are purchasing via Steam DOES use SecureROM or some other form of DRM, then that's not Steam's fault. That's how the publisher wanted it.
But overall, Steam itself is a great program and doesn't itself install anything malicious. I think it still has a bad rep because of what happened at the launch of HL2, but you know what? That was the first of it's kind and things like this always trip up at initial launch. Now, how dedicated the company itself is to getting the kinks worked out is what decides whether it is a success or failure, and right now, Steam is a major success and I like it.
And, unless the game requires online play, you can set Steam to be in offline mode and as long as all your games have a status of being up-to-date and they don't require online connections to play, you don't have to be online to play your games via Steam.