Guilds are one of the key features of this game, and not being in a guild means you are missing out on a lot of the game. Whenever you raid or are raided, attack/defend a fort its the Guild Army units which come to your rescue.
How the guild army works is that each player can contribute 1 of their units to the army. This unit is then sent to fight in other member's battles whenever their level matches up. This makes it so that the probability of someone winning an attack can depend as much on the strength of the guild army reinforcements as the strength of his own units. This is why its important to put maybe not your best unit, but definitely 1 of your top 3-4 units in the army. If everyone puts weak units in, it just makes everyones battles much harder.
You don't lose LifePoints for army units, and they don't lose weapon durability. They also gain a tiny bit of exp every tick (8hrs). If you have a strong character how has low lifepoints, or a powerful weapon you can't repair which is nearly destroyed, putting them in the guild army is a decent strategy. However, if you do put a nearly broken weapon on a guild army unit, you should make sure that it has at least 8-10 durability left. Even though it never loses durability on your screen it still gets *temporarily* used up in battles. So a weapon with 1 durability means that it will only attack once per battle.
Listed below are some of the active guilds in the game.