I have included two videos from the Google Talks series in this post which I think will help you to really understand, not just use the language. I have learnt a lot from these talks, and I hope you will do to. So without any more waffling, here they are.
Libraries are a huge part of software design and engineering. You and me both use them all the time without even realising it. Do you have a core set of (even simple) functions which you include (or copy-paste) into your new projects? That‘s a library. Strictly, it’s only a library if you include it as a whole, but loosely, a copied and pasted function could be described as a library.