The first major expansion for Civilization V is Gods & Kings, released in 2012. Civilization V is a turn-based strategy title in perhaps the most well-known turn-based strategy franchise in video games. While Gods & Kings does add some material content like units and new civilizations, the focus of the expansion is on two new major background mechanics -- religion and espionage.
Additionally, Gods & Kings adds a few scenarios to extend the campaign content of the game: the Fall of Rome, Into the Renaissance and Empires of the Smoky Skies. The Mongolian campaign previously provided as free DLC was also made available all in one package with the expansion.
As with the main game, Gods & Kings is a turn-based strategy game where players build and expand a gigantic civlization through various means -- technology, culture or even military might. Gods & Kings adds in two mechanics, that while aren't as large of contributions to victory as the other main features, still manage to change the pacing and direction of the game in significant ways.
A religion can be adopted fairly early in the game, beginning cultivation of the faith resource. Using faith, a player can upgrade their religion to provide various benefits, including the production of Prophet, missionary and inquisitor units. Religion in the early game can heavily influence relationships with other nations, leading to truces with nations who share your religion, and new battles with opposing religions. As nations approach the modern age, religion tapers off in effectiveness.
Espionage, as it implies, focuses on sending spies to perform various actions in other nations. If successful, spies will level up and perform more complicated actions, though their survival is largely based on randomness. Espionage is meant to increase in effectiveness just as religion fades.
Gods & Kings adds religion, espionage, several new scenarios, as well as nine new playable civilizations.
 Hardware Info
Civilization V's Gods & Kings expansion is available on PC, Mac and Linux.