Libraries are collections of code that are intended to be usable by multiple consumers (if you're interested in the etymology, watch this video). In the old days we had what we now refer to as "static" libraries, collections of code that existed on disk but which would be copied into newly compiled binaries. We've moved beyond that, thankfully, and now make use of what we call "dynamic" or "shared" libraries - instead of the code being copied into the binary, a reference to the library function ...