Plugins are separate programs that when properly installed become part of Premiere Pro.
Templates are project files that can be used by substituting the original media with your own media.
Presets are saved effect parameters that can either be included with Premiere Pro or installed to Premiere Pro.
Motion Graphics Templates are graphics (or text or both) templates created in After Effects. The Mogert is then imported into Premiere Pro where the user can change any parameters or replace objects, providing that the Mogert creator configured the project to allow the changes.
The user can also make changes if the project is opened in After Effects and can be saved as a new Mogert.
The typical uses for Mogerts are titles, lower thirds, and animations.