Each ticket can have multiple people involved. Most times you will have just a Reporter and an Assignee. Here are the people you might see:
Assignee: The person tasked with resolving the issue.
Reporter: The customer who sent the request.
Request participants and Organizations: Customers and groups of customers who can view and comment on the issue. This is a new field that will be useful.
Votes: People who vote for an issue are people who want the issue resolved.
Watchers: Team members in your project who receive notifications about the issue.
Approvers: If the issue has approvers, this field displays people who are tasked with approving or declining the request.