WordPress Core File Structure

Hey People! 🙂

Iam a software developer interested in contributing to the core of WordPress. I have already set everything up on WAMP and have everything running and have installed WordPress on my machine

One question I have is how the code is structure. I already know of the main folders and what they are used for like wp-admin (main for backend admin code) and wp-content (for front end content).

However, what is the wp-includes generally used for?

Also, I can see that all 3 folders have js folders that are ignored by SVN. Am guessing these contain 3rd party code and JavaScript plugins. If so, where do we add custom js code?

I know this are newbie questions but I need to get the lay of the land before I can look at code at a deeper level.

Thanks Guys 🙂