What are FTL files

Freemarker template files I believe.

http://freemarker.org/

FTL stands for FreeMarker Template.

It is very useful when you want to follow the MVC (Model View Controller) pattern.

The idea behind using the MVC pattern for dynamic Web pages is that you separate the designers (HTML authors) from the programmers.

'ftl' stands for freemarker. It combines server side objects and view side (HTML/JQuery) contents into a single viewable template on the client browser.
Some documentation which might help:

http://freemarker.org/docs/

Tutorials:

http://www.vogella.com/tutorials/FreeMarker/article.html

http://viralpatel.net/blogs/freemaker-template-hello-world-tutorial/