Counter-Strike: Global Offensive is not exactly a sequel, but rather a refinement of what is regarded by many people as the best competitive online FPS ever made. It's a fantastic update to a genuine classic, and essential gaming for Windows and Mac.
The basic idea of Counter-Strike: Global Offensive is two teams, terrorists and counter-terrorists fighting each other in a variety of maps, with a secondary objective, such as hostage rescue. There are classic game modes, where you fight in rounds of short games. Death is permanent in each game, so if you die, you sit out until the next game. There is no XP (experience points) upgrading like in Battlefieldetc, making this a purely skill based game.
There are also two new modes, Arms Race and Demolition. The former introduces re-spawning to Counter-Strike, so loses the tension of classic modes, in favor of Call of Duty feeling fast-paced gaming. The permanent death modes are much better though, as the need to stay alive is what gives Counter-Strike: Global Offensive it's unique and highly addictive flavor.
Counter-Strike: Global Offensive is still a hardcore FPS. It's highly competitive, and the classic maps mean that experienced players have a big advantage. New players can expect lots of quick deaths, but being able to watch other players when you're dead means you can quickly pick up on tactics and learn the maps. Despite the toughness, Counter-Strike: Global Offensive feels much fairer than many modern games where you can feel out-gunned by players with more XP than you.
While the basic gameplay is the Counter-Strike we've been playing for 12 years, the graphics have had a welcome polish, there are some new maps, and the old ones have been slightly tweaked. Jumping into a game is really simple, and the new scoreboard is excellent, as is the improved HUD (Heads-up display).
Counter-Strike: Global Offensive is a great update of a classic game. If you're willing to put the effort into building your skills, it's possibly the most rewarding multiplayer FPS game you can play.