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.