Go (common meaning: "encircling game") is a board game for two players that originated in China more than 2,500 years ago. The game is noted for being rich in strategy despite its relatively simple rules.
Go is an adversarial game with the objective of surrounding more territory than one's opponent. As the game progresses, the board gets divided up into areas of territory, as outlined by groups of stones. These areas are then contested in local battles, which are often complicated, and may result in the expansion, reduction, or wholesale capture and loss of the contested area. It is often the case that a certain kind of "trade" goes on, where a player's loss in one part of the board can be compensated for or mitigated by a gain in another part of the board.
A basic principle of Go is that stones must have at least one "liberty" to remain on the board. "A liberty" is an open "point" (intersection) next to a stone. An enclosed liberty (or liberties) is called an "eye", and a group of stones with at least two separate eyes is said to be unconditionally "alive". Such groups cannot be captured, even if surrounded. "Dead" stones are stones that are surrounded and in groups with poor shape (one or no eyes), and thus cannot resist eventual capture.
The general strategy of Go is to expand one's territory where possible, attack the opponent's weak groups (groups that can possibly be killed), and always stay mindful of the "life status" of one's own groups. The liberties of groups are countable. Situations where two opposing groups must capture the other to live are called capturing races. In a capturing race, the group with more liberties (and/or better "shape") will ultimately be able to capture the opponent's stones. Capturing races and questions of life and death are examples of what makes go challenging.
The game ends when both players pass, and players pass when there are no more profitable moves to be made. The game is then scored: The player with the greater number of controlled (surrounded) points, factoring in the number of captured stones, wins the game. Games may also be won by resignation, for example if a player has lost a large group of stones.