What is an Email Template Builder?
It is tool that helps you to build emails, whether it is with a template (E.g. Beefree) and modules system or just coding (E.g. Litmus).
They usually have a drag-and-drop interface also refered as What You See Is What You Get (WYSIWYG).
Email Service Providers have created their own builders but they usually lack features or simply not the best.