A library is a set of pre-written code - functions, classes, etc. - packaged in a way that can be used by many other projects. C++ comes with its own standard library, which includes things like strings, vectors, and even cin and cout - this functionality isn't directly a part of the core C++ language, so we include other libraries to use this functionality.
We can also find other third party libraries online to help us do stuff like advanced math, user interface display, playing media, connecting to databases, and more.
To use these libraries, we have to download them, and link them to our projects. We will step through this process here.