Document Libraries are a high-level structure used to manage files. Document Libraries have the added benefit allowing access or security settings to be easily applied.
For best practices of when to use a Document Library, please see the FAQ. ( THIS LINK NEEDS TO BE UDPATED TO THE KB FAQ)
Adding a Document Library
To create or add a Document Library to your site, first navigate to the Settings-Gear and click the Site Contents menu option.
Your screen will change to show the site's contents. Left-click the plus sign ("+") near the top-left to show the New menu options. Select the Document Library option. You can choose between starting out with a Blank Library, or a Library based on a preexisting Document Library. In this instance, select the Blank Library option.
You will then be prompted to enter the name of the new Document Library, as well as whether you want the Document Library to show up in the site navigation. To be consistent with how your site has been created, uncheck the "Show in site navigation" checkbox.