What is the difference between a template email and a segment email?
A template email is one that you are probably going to send automatically, usually as an action in a campaign, from a form trigger or a points trigger. With a template email you can create the email and use it in all of those scenarios, but you can't do a manual send to a contact segment.
Think of these as your 'automated emails' which you will be using to send transactional communication.
A segment email is what we traditionally would call marketing emails - or broadcast messages. They can be manually sent to segments, and importantly each contact can only receive the mail once. Think your traditional newsletter email blasts.
You can read more about this and other useful tips on the Mautic Documentation.