What's the difference between binary and executable files mentioned in ndisasm's manual?

I want to compile my C file with clang and then decompile it with with ndisasm (for educational purposes). However, ndisasm says in it's manual that it only works with binary and not executable files:

   ndisasm only disassembles binary files: it  has
   no  understanding  of  the  header  information
   present in object or executable files.  If  you
   want  to disassemble an object file, you should
   probably be using objdump(1).

What's the difference, exactly? And what does clang output when I run it with a simple C file, an executable or a binary?