Dungeons & Dragons Online: Eberron Unlimited Developer Diary

In a new developer diary on the official Dungeons & Dragons Online website, Turbine's Ian Currie and Erik Boyer describe the mechanics behind the new guild leveling system that will go live with the game's forthcoming Update 5 content patch. The gist of it:
So once Update 5 goes live, every guild will have a Guild Level. This Guild Level will be visible to all and default to 1 for every guild at the start, whether new or old. As guild members adventure, they'll increase their level by increasing the guild's renown. Think of your renown as your guild's fame, or reputation. The more valiant your efforts, the more your tale is told in taverns! As your renown grows, so does your level, and thereby, so do the reward opportunities available to your guild.

Renown can be increased by finding various renown (trophies.) These items will show up in random chests and drop from monsters, and can be offered as rewards from quest givers. For those who want to expedite their guild's progression, they can focus on tougher challenges; the more difficult adventures a character attempts, the more likely it will be that renown is earned. That said, there is only so much a guild can progress in one day (after all, there are only so many songs the tavern bards can sing about your trials before everyone's heard the news). This means that after a certain amount of daily progress, you'll have to work harder to produce more renown.

Guilds level with renown, much like characters level with XP. However, unlike character leveling - which can only increase - a guild's level can go down in some circumstances. This is known as renown (decay).