Templates are preset "effects" settings that you can simply drag and drop onto any track to alter the sound.  You can click on each one to get a sample of the sound/effect it has.

"Track Synthesizers" are simply "virtual instruments" usually in a form analogous to any number of real, hardware synthesizers and some that are unique.  They are tools/instruments available for YOU to compose and create your own compositions/melodies/pads/rhythms, etc.  You can also add to the pool of instruments by installing any number of free or purchased VSTi's (Virtual Studio Technology instruments).

Here is a shot of my current pool of instruments - some that came with my two versions of MMM and many that are free VSTi's that I've installed.