Anything related to media stored in the database or file system and created as items in the Media Library.
You can manage all your media items from the Media Library, such as images that you want to embed in a webpage or make available for download by visitors. The Media Library contains all you media items, such as images, documents, videos, and audio files.
In the Media Library you can:
- Keep all your media files in one place and organize them in a folder structure similar to the content tree.
- Navigate the content tree or use Sitecore’s search functionality to quickly find media items.
- Upload media files to the Media Library. You can use the upload buttons to browse your computer for images or simply use the Drag & Drop dialog box to add media files.
- Make every media content item and media file versionable.
https://doc.sitecore.net/sitecore_experience_platform/content_authoring/media_items
