Plugin development: Forms
There currently aren't any docs for _includes/forms... you'll need to backwards engineer it. You'll notice some common patterns as you dig in there, which gives great insight into the overall Craft structure.

That file (or batch of files) is one of the most commonly backwards engineered parts of the core code. You may be able to find other posts (on Stack Exchange or blog posts) which describe it in some detail, but you'll still want to investigate on your own.