Its a demo where Yanfly has several scripts already included. The demo revolves around how to use the scripts.
I believe it was made for beginners to have a base to start building on a game. I've downloaded once, and have been searching for it, but I can't find it.
Forgive me if this is in the wrong section.

A demo showing all of yanfly's scripts? I'm not sure one exists, but you could always skim through all of the posts at the website.

http://pockethouse.wordpress.com/

There are bundled scripts (such as the core battle redux and additions) but I am not sure if that's what you mean. You might be thinking of the KGC script compilation which showcases all of the translated scripts. I don't think a complete collection of yanfly's scripts exist, or at least on that site.

Link to compilation of Yanfly Engine Zealous is in this link (link to RPGMakerVX.net).

1) Go to the link Sizux posted
2) In the search bar, search for "Subclassia"

It's a demo that doesn't contain all of his scripts, but it does contain a number of scripts and explains the class and sub-class scripts. And it's the only YanFly demo available.

Thanks Sizux! I was looking for the KGC script compilation demo.