A guild is a virtual group within Catan Universe that can be joined by people sharing the same interests. For example, there are guilds for people from certain countries, people with certain world views or fans of a certain variant of Catan (like Rivals). Within the guild channel you can communicate with your guild colleagues, arrange joint games etc... Some guilds even have an own forum within the Catan Universe forum.
You find a listing of all games when you click on "Guilds" within the ingame menu.