SRC contains how page will appear in browser and code behind contains server side code required for page processing

SRC specifies the source file name of the code-behind class to dynamically
compile when the page is requested.

CODEBEHIND specifies the name of the compiled file that contains the class
  associated with the page. This attribute is used by the Visual Studio

We use SRC  when we use notepad or other text editor tool.Code behind is relevant to Visual Studio only.