We are glad that we have finally achieved a rather stable implementation of the guilds and claims system! From now on, you will be able to build, and later upgrade, a special monument, which will create your guild and will claim an area around it to protect anything that is built there from EVERYONE except members of that guild. To learn more about claims mechanics, we suggest you to take a look at the updated wiki page here: http://lifeisfeudal.gamepedia.com/Claim
The decay system has been improved to take claims into account. If your building is FULLY covered by your guilds claim, it will be completely protected from decay. However, if ANY of the building cells are outside the claim, your building will start to decay and eventually will disappear in 2-3 in-game days. However, decay rate setting is set to 0 (no decay) by default on all servers, so it is up to you to change it or not.