Perfect timing, NSM.
I got this a week or so ago and it's already stolen 33+ hours of my time, so... yeah. Fabulous game.
The basic premise is that you are a world leader (from a list of twenty or so countries) who starts the game by building a city. As the game continues, you can improve your land, build warriors, explore territory, create new cities, etc. The goal is to make your civilization the most successful possible, so you've got to keep your citizens happy, keep developing new technology, and keep defending your territory.
The first game I played (ended up taking 25 hours in total) I was Otto von Bismarck and I managed to conquer all of East Asia, Australia, and the Pacific before I won by developing enough technology to build a spaceship. However, you don't have to play such giant, long games - there are plenty of options you can choose to speed up the gameplay and shrink the map so you don't have to conquer every single inch of the planet to win.
The best part of the game imo is how adjustable it is. Do you want to build the most advanced civilization possible? Go ahead, you can win doing that! Want to conquer all your enemies and rule the entire world? Knock yourself out. Want to have a small but cultured nation? You can do that too.
If you're a fan of any strategy games and/or building games, this is a must-buy. If you want long-drawn out games (like me!) or short, fast-paced ones, you can easily change the settings to do that. Heck, if you just like history (like me!) then this is also entertaining just because you can build whatever historical soldiers you want, especially since you can choose to start in the Ancient, Classical, Medieval, Renaissance, Industrial, Modern, or Future Eras.
A word of caution: it is pretty complicated. As in I would probably run out of character space explaining every single possible thing you should be considering. Fortunately, the game's pretty good at giving you tips on how to run the game and there's a list of advisers in-game who will suggest building this or that, so if you're unsure of what to do the game will help you out.