Assets, based on their location, can be classified as:
Source Assets
Source asset files are available together with PHP source code which is not accessible directly via the web. To use source assets in a page, they should be copied to a web directory and turned into the so-called published assets. This process is called asset publishing which will be described in detail shortly.
Published Assets
Published asset files are available in a web accessible directory and can thus be directly accessed via the web.
External Assets
The asset files are located on a Web server that is different from the one hosting your Web application.
Reference: Yii2 Doc