Libraries are where you go to manage your documents, music, pictures, and other files. You can browse your files the same way you would in a folder, or you can view your files arranged by properties like date, type, and author.
In some ways, a library is similar to a folder. For example, when you open a library, you'll see one or more files. However, unlike a folder, a library gathers files that are stored in several locations. This is a subtle, but important, difference. Libraries don't actually store your items. They monitor folders that contain your items, and let you access and arrange the items in different ways. For instance, if you have music files in folders on your hard disk and on an external drive, you can access all of your music files at once using the Music library.