The correct answer to this question is both A and B. An email template is typically used when you often write generally the same email. For instance, universities may use email templates when they send out acceptance letters. Usually they would just use auto fill to fill in the potential student's name.
A sales person can also use an email template if they usually send out the same email to potential customers. A template is set in stone until you are to go in and change it, but once you change it, it will not affect other emails until you save the template.
Emails that have been created from a template are not automatically updated when the template is edited. An email that is updated to reflect changes made in its template can be found in the email properties.