The "official" campaign is nothing more than a series of offline matches of basic counter-strike where you play against bots. Each mission is just a different map (with favorites such as Office, Dust 2, Inferno, etc). In order to get to the next map you have to kill a certain number of enemies with a certain weapon, rescue X number of hostages, or some other mundane challenge. The campaign is pretty easy, short, and definitely not worth playing.
Deleted Scenes is the reason this game is worth trying. It's still not a great game, and probably not even worth finishing because it gets awfully repetitive towards the end. It still is a proper Counter-Strike campaign, though. The campaign is once again a series of unconnected missions, but this time each mission actually has a story that you linearly progress through. The missions take place all over the world, and you always play as one of that region's counter terrorist forces. Some of the missions include surviving a helicopter crash in a terrorist controlled middle east, recovering a lost nuke in arctic Russia, or destroying drug labs in Columbia. Overall each mission is decent enough, and the awesome counter strike gunplay combined with new tools and gadgets helps make them enjoyable, but far from perfect. My biggest complaint is that the game is incredibly cheap with where enemies spawn. You need to check EVERY corner, because there is an extremely high chance an enemy will be standing there silently waiting to shoot you in the back if you run by without checking. Enemies will also spawn in areas you've already cleared, so be prepared to get shot in the back a lot. Finally, enemies are unfair with how fast they can spot and shoot you. I'm convinced they're tracking me through the walls and ready to fire the second I appear in line of sight.