The tag covers transactional emails and everything directly related to it such as invoices, orders and variables used in the emails as well as the formatting and configuration of these emails. See also: email-templates
Transactional emails are emails sent by Magento using a template which is managed in
System > Transactional Emails. Usually, transactional emails contain customer data that is dynamically assigned to the template.
For questions about creating/modifying/assigning email templates, use email-templates