Open Source Libraries

I might have given this the wrong name, but if you know how to add highlight.js, jQuery, or any other things like that. Then how do you create your own library?

For example, say I want to use a background on a website, but I would like to create the background so I can use it on other websites. Creating it in a way like using hilight.js. You add the library to the HTML file, call the highlight.js function, and give the element(s) you want to highlight a class or surround it with a <pre><code> tags.

Thank You!