The SRC and HREF attributes are used to include some external entities like an image, a CSS file, a html file, any other web page or a javascript file.

Is there a clear differentiation between SRC and HREF? where/when to use SRC or HREF? I think they can't be used interchangeably.

I'm giving below few examples where these attributes are used:

To refer a CSS file: href="cssfile.css" inside the link tag

To refer a js file: src="myscript.js" inside the script tag

To refer an image file: src="mypic.jpg" inside an image tag

To refer another webpage: href="http://www.webpage.com" inside an anchor tag