Guild tasks are communal tasks that all members of a guild can contribute gold to. Upon completion, all members of the guild will earn the rewards from that task (and the guild, along with the appropriately-colored guild statue, earn some XP).
Each player can contribute gold in hundreds or thousands, even if they can't finish the task themselves. Once a task has been completed, the rewards will show up in the mail for everyone in the guild.
There are two kinds of tasks: basic, and legendary.
Basic tasks are what all guilds will be completing. There are six different task chains, each providing their own sets of rewards - for instance, the purple task chain provides Souls. A chain has twelve tasks, with each task in a chain becoming increasingly more expensive but yielding greater rewards.
Upon finishing all twelve tasks in a chain, an additional temporary reward is unlocked: a skill bonus to your troops that lasts for seven days. These can stack with bonuses unlocked from the other task chains, but not with themselves.
Legendary tasks are limited to the highest-earning guilds and are only available once all six basic task chains have been completed. Each legendary task requires a whole 1,000,000 gold to complete, and earns the guild members a set of three random rewards.