Tasks become available once a character has joined a guild. Tasks are a completely voluntary system and are not required for anything and are provided as a mechanism to vary up your gameplay and provide opportunities to do something different while out adventuring.
Tasks are given out by taskmasters, NPCs that dole out requests for service in exchange for rewards.
Repeatable vs. One-Shot Tasks
Some tasks are repeatable after a cooldown while others can only be completed once.
The cooldowns on tasks vary depending on the rewards as well as the difficulty of the task, or sometimes just based on whim.
Most commonly, tasks that are repeatable can be attempted again after twenty-four hours.