Assets are files that can be managed through the Pimcore CMS. The most common assets are images.
In Pimcore you can organize assets in folders. Some file types, like images can be edited directly Pimcore, and can be used to create thumbnails.
Other kinds of common assets are PDF or MS Word documents which people can download from the website.
Page:
Asset Lists
Page: Custom Settings (Properties)
Page: Image Thumbnails
Page: Video Thumbnails (since 1.4.3)
Page: Custom Settings (Properties)
Page: Image Thumbnails
Page: Video Thumbnails (since 1.4.3)