What is the difference between a requestor, a watcher, and an owner in an RT Ticket?

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A watcher is someone who is interested in a ticket. You'll find these filed under "People" when you're looking at a ticket. A watcher of a ticket normally gets email-copies of all the replies to the ticket. The watchers who are staff-members get copies of the comments too. There are several types, or roles, of watchers:

• OWNER: the person responsible for the ticket and its resolution. Each ticket can have only one owner.
• REQUESTOR: the person or people who asked for something to get done; the ticket's reason for being. The Requestor normally gets copies of all replies to the ticket, but no comments.
• CC: someone (or someones) who should get copies of any replies that go to the requestor. This might be the requestor's boss, sales rep, etc. This person will see the email but doesn't necessarily have the right to work on the ticket.

Four other roles are listed on tickets, though they frequently will appear blank.  These roles are more commonly assigned across an entire queue, but some tickets may make use of these.

• ADMINCC: A user with full administrative rights for the particular ticket.  This user may add correspondence and comments, change values of the ticket like the subject or custom fields, add or remove people from the ticket, or become the ticket's owner.
• L1: A user with limited administrative rights for the particular ticket.  This user has most of the same privileges as the AdminCC role, except they cannot become owner of the ticket.
• L2: A user with viewing access for the particular ticket.  This user may view all of the ticket's information even if they do not have access to the queue the ticket is in. 
• QCC: A user who will receive notification emails of all updates to a ticket, but only if the ticket does not have an owner assigned.  If a user should always receive email notifications, they should be added to the CC role instead.