An excerpt from http://www.sitepoint.com/javascript-and-php-libraries-used-by-wordpress/, by @AleksanderKoko
Almost all modern software (including platforms such as WordPress) rely on third party libraries. That’s because, as developers, we don’t need to reinvent the wheel.
If I had to develop software that uses HTTP abstraction, I might use a package like Symfony httpKernel or similar. If my software needed more than that (unlikely by the way, but let’s imagine that for our example), I could extend that package, or in the worst case, create one from scratch. Creating something from scratch isn’t a bad thing. It can actually be a good thing, since it makes you a better developer. But commonly, in most modern projects, you’ll have to depend on third party libraries (or packages). In this article we’ll take a look at the various third party JavaScript and PHP libraries used by WordPress.