In Zendesk, tickets are Solved first and then after they have maintained their solved status for a predefined amount of time (set up in your automations or triggers) they are moved to Closed.
The major differences between Solved and Closed tickets are that:
Solved tickets can be reopened and updated. Tickets can be marked Solved by Agents or End-users. Tickets marked as Solved and responded to by an end-user will be reset to an Open status.
Closed tickets are locked. They cannot be re-opened or altered. They can only be Closed via a Trigger or Automation.
You can create a "Follow up" from a Closed ticket that will include a link to the Closed ticket. Please note that tickets cannot be closed manually; they can only be set Closed via an Automation or Trigger (or the system batch close, discussed later).
Your Zendesk trial comes with an Automation called "Close ticket 4 days after status is set to solved". This Automation allows your end-users 4 days to respond to a Solved ticket before the ticket will be moved to a Closed status. You can set this Automation to run anywhere between 1 hour and 28 days.
Best practice guidelines recommend that you leave a ticket in Solved status for 3-5 days before moving to Closed. This allows your end-users to re-engage with you, and their answers will still be threaded in the same ticket. If you move the ticket to a Closed state and the user has follow-up questions or needs clarification, their response will open a new (follow-up) ticket rather than thread in the original ticket.
If you deactivate the 'Set to Closed' Automation, your Solved tickets will be set to Closed via a system action (noted in your events and notifications as a 'batch close') after 28 days of inactivity.