Guilds are a group of players that can conquer content together. Since much of the content is cooperative, it makes sense to join a strong guild. Each guild is part of an empire and each empire owns one of the four cities in the realm. Players can only attack players from other empires.
There will be guild specific events where the top guilds will be rewarded well. Rewarding cooperation is a consistent theme in WEO and guilds are a primary part of that.