Tasks unlock at level 5 and give you in-game currency and other rewards. Tasks are of 3 types: bonus, everyday and clan tasks.
Daily tasks are general missions with goals like capturing or liberating beacons, dealing damage, winning battles, etc. that reward you with Au (gold). Two new tasks appear every day unless there are 4 or more uncompleted tasks—the list can have up to 5 tasks at a time. For example, if you have 4 uncompleted tasks you will get only 1 new task instead of two.
Bonus tasks are mainly event specific missions, Facebook related, payment or video ad tasks. The type of tasks and rewards varies.
Clan tasks become available once you create or join a clan and take into account achievements of all your clan mates.
NOTE: You can add progress to some of the tasks by watching ads.