Libraries are Sketch documents containing components that you can share with multiple teams, and reuse across all documents.
Libraries help you organize your components (Symbols, Text Styles, Color Variables, Layer Styles and Artboard Templates) and reuse them across documents. When you update components in a Library, any documents using these components will get an update notification. You can review those changes and confirm them to make sure you’re always using the most up-to-date components in your work. You can even enable a Library for a single document, if you prefer.
Libraries are handy for teams working on designs that share components like icons, UI elements or branding. But they’re just as useful if you’re working solo. With Libraries, you can quickly insert common components (like Apple’s iOS UI elements) into your designs and be sure they’re always up to date.