In the tasks app you're able to assign tags to tasks, so that when a task is completed, the ticket is given that tag. By adding a tag to the ticket, this taps into the power of Zendesk triggers, allowing you to spin off any workflow you like as a result of the task being completed.
Here's a video explaining how it works:
To attach tags to tasks, you will first need to make sure you've got the feature turned on. You can do this by clicking on the tasks icon in the main menu on the left. Then click on 'Settings'.
Then make sure the box is ticked next to "Tag ticket when tasks complete" and click "Save settings" at the bottom.
Now click on "Task Lists" in the main screen.
Here you will see there's a whole other column when you build a list. This is where you're able to set the tag that will be added to the ticket when the task is marked as completed.