This is basically a more intense version of the SF PvP you might do. Of course with guild wars, instead of a factional battle you have two guilds fighting each other head to head. This type of PvP can be much more fun, seeing as you don't wait for people to go overt as the enemy guild will actually remain hostile, giving you non-stop action. You can fight in your city or even within theirs. To begin a guild war, the leaders of both guilds need to go to the guild terminal and, under enemies, you can add or undeclare on previous enemies. There are signs to the panel, when the enemy guild has declared you will see a < sign. When you have declared you will see a >. When both sides has combined there will be a =, this is how you will know that the opposing guild is hostile.