The User object represents a User.

Groups are a collection of users in the source application. In some applications, they might also be called Teams.

Accounts represent the companies or organizations that you are in contact with. Accounts have one or more Contacts associated with them.

Attachments are the files associated with a ticket or a comment.

Comments represent the communication happening on a Ticket, both between a User and a Contact and the internal things like notes, private comments, etc. A Ticket can have one or more Comments.

Contact represent the external people you are in contact with. These could be customers, leads, etc. Contacts can be associated with an Account if the underlying product supports it.

Tags represent a common classification approach used in various ticketing systems. A Ticket may have one or more Tags associated with them.

Teams represent the grouping system used for Users. These are usually called groups, teams, agent groups, etc. in the underlying products. A User can belong to one or more Teams.

Core resource which represents some work that needs to be carried out. Tickets are usually mapped to issues, tasks, work items, etc. depending on the underlying product.

Ticket Status represents the completion level of the Ticket. Some products provide customizing the Ticket Status.

Users represent the people using the underlying ticketing system. They are usually called agents, team members, admins, etc.