In the paid subscriptions you will be able to create more than 1 board. A board is a set of lists.

The additional boards are Shared Boards. meaning that you can share the board with someone else (who also has a paid subscription). The additional boards are only shared when you invite someone, so they can remain private if you so wish. On Shared Boards you can then assign a task to the people who share the board. This functionality is not generally available yet, so things will become a bit more apparent when we release the functionality.