Transactional emails are emails that are sent to a user automatically as a result of taking a certain action (or when certain actions are not taken).
If you have ever forgotten your password to an online website or application and clicked on the “Forgot Password” link, you automatically receive an email in your email inbox with a link to reset your password.
Well, this is an example of a Transactional Email. Clicking on the “Forgot Password” link was the action that triggered that email.
Alternatively, if you added a product to your online cart but did not make a purchase (did not complete an action), you may receive an abandoned cart email reminding you of the product.
There are many examples of transactional emails, here are some:
Confirmation email after you sign-up on a website or online service
Email receipt of a product after your purchase
Weekly account summary emails
Notification emails (e.g. when someone follows or retweets you on Twitter)
Abandoned cart emails
The other type of emails are “Marketing Emails”, the kind of email that you generally associate with email newsletters, promotional product emails etc.