Can I add relationships between backlog items or tasks?

Yes, you can add relationships between backlog items, tasks, releases, and sprints. For example, to add a relationship from a backlog item, open the popup dialog for the backlog item, click the "More" link at the bottom of the dialog, and then click "Add a relationship":

You can also use the right-click menu instead. For example, select two backlog items, and then select "Add a relationship" in the right-click menu.