On second thought, perhaps you meant "What is _A_ message header".

Inside emails, there can be many sections, each with a 'header-id-code'. You won't necessarily visibly see all these sections - some can be just the pure ASCII text of the message, some can be HTML encoded versions of the message text, some can be Word mungles of the text, some can be executable code, some are technical routing information. It will depend on your email package, and how you have set the options as to what you will know is there. With the right tools, you can peer inside the full message - MailWasher is one of these tools which is intended to look at your email and filter/delete/bounce things before you download it into your normal mail package.

Robin