The accounts represent a company in a CRM.

The contacts represent an existing point of contact at a company in a CRM.

The engagements represent an interaction noted in a CRM.

The opportunities represent an opportunity in a CRM.

The tasks represent a task in a CRM.

The users represent a user in a CRM.

The User object represents a User.

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.

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 Types represent the classification system used by the underlying products for Tickets. Some examples are bugs, feature, incident, etc.

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

Ticket Priorities represent the intended order in which the Tickets should be worked on. Some products provide customizing the Ticket Priorities.

Workspaces represent the top-level subdivision in a ticketing system. They usually have their own set of settings, tickets, statuses, priorities and users. Some of the usual terminologies used by the products for the top-level subdivision are projects, bases, spaces, workspace, etc. A Workspace could belong to an Organization.