The Inbox is where new Stories (and Issues/Ideas or impediments/improvements) are put when created from the main menu.
A user can have different site and circle access right which will determine what they can or cannot do.
Users are invited by email. The user invite specifies the roles and/or team memberships of the user.
A Preplanned list is an ordered list of product backlog items that are yet to be explicitly planned. Preplanning stories/epics is to obtain an early grouping of items for a future release.
A team is a collection of users. A team can be mentioned in comments and assigned to tasks.
A site is an organization account.
The milestone specifies a deadline. Epics and stories can be linked to this milestone to indicate that they must meet the deadline.
A Sprint is a time-box during which a “Done”, useable, and potentially releasable product/service increment is created.
A Release is a special type of Milestone. It is the base for the planning in the Product Backlog.
A circle is a unit within the organization containing one roadmap and one product backlog.