There is simply no better world simulation yet to be created.
To start, Civilization IV is the continuation of a long running series from Sid Meier. This game is a world simulation game that allows you to start your own civilization from the stone age, through to future ages.
Your objective is to lead your civilization to victory. What makes this civilization better and different is the fact that you can win the game through carious means such as being the cultural leader by collecting enough cultural points. Another one is to conquer the world like an evil emperor or you can be the first to travel into space. With so many ways of winning, it's really up to you how to play.
There's dozens of civilizations to choose from and many of them have various leaders. If you like Napolean of the French, that's great, but if not, you can choose Louis the 14th. Each leader also has different bonuses to how you play. Some increase your money while others increase the growth rate of your cities.
The terrain of the world plays a significant role. Certain types allow roads to be built better while sea spots have extra food available. Your workers are able to build roads, other cities and other key pieces of infrastructure. For example, they could build a mine, which gives you a chance to find iron. Without iron, you can not build swordsman, an early offensive unit. It is key for you to continously extend your reach, otherwise, enemies will be invading your lands, stealing your resources and encroaching on things that were once under your influence. The great example of this is the fact that you can become so influential, you can take over cities without ever laying a figure on them because of your sheer influence.
Never being the same, keeps the gameplay fresh almost every time. With so many different mods and playable scenarios, it's nearly impossible to predict what your opponent is going to do next. This is exactly the same in Multiplayer. The game does allow you to have simultaneous turns so does speed it up a bit.
Technologies still play a large part of the game and researching them allows you to get other technologies and new units. Trust me, a battleship will destroy a frigate any day of the week but you better have enough oil to keep your army going or it won't matter what technologies you have.
One key part of the game is too make sure to make allies with some neighbours while making enemies with the others. Montezuma of the Aztecs is extremely untrustworthy and very militaristic so getting them as an ally or getting them to do what you want is nearly impossible at times. Civilization has so many layers, that everyone may find something to like. I recommend this game whole-heartedly.