The Guild Wars was a period of time in the history of humans of Tyria. They were wars between different Guilds in the human kingdoms of Orr, Ascalon, and Kryta. The fighting ended when the Charr invasion drew the human races together in defense against a greater threat from the north.
The Guild Wars lasted for over 50 years (from 1013 AE until 1070 AE), causing much suffering and hatred between the inhabitants of the kingdoms. These old wounds are still evident, most notably in King Adelbern's mistrust of Kryta's willingness to help Ascalon recover from the Charr invasion.
There have been many Guild Wars, but when characters in the game speak of the Guild War, they refer to the most recent war.