Headers - What is a website header?

The header is the top rectangular shaped area that runs across the top of the web page on your screen.

The primary purpose of your header is to promote your company’s brand and make it instantly recognizable to your audience.

Your audience should instantly know they have arrived at the correct place, even if they are not already familiar with your company.

For some companies where the brand is less important you should try to convey what the website is about and what the website does by using the header. This can be done with the use of short text or a relevant graphic, which conveys the general concept of your site/product – for example, a company that manufactures bicycle clothing might use an image of a cyclist in action.

This header is the most valuable space on the screen. Or, in designer’s terms – it is the most expensive piece of screen real estate. The header should always be used wisely, keeping in mind the goals and vision of your web site design.

1) In a document , a header is some combination of text and image's that can be made to appear at the top of each page when displayed or printed.

2) In e-mail , the header is the part of a message that describes the originator, the addressee and other recipients, message priority level, and so forth.

3) In a computer file, a header may be a field that precedes the main file content and describes the length of the content or other characteristics of the file.

4) In a network transmission unit, a header precedes the data or control signals and describes something about the file or transmission unit, such as its length and whether there are other files or transmission units logically or physically associated with this one

In information technology, header refers to supplemental data placed at the beginning of a block of data being stored or transmitted. In data transmission, the data following the header are sometimes called the payload or body.

It is vital that header composition follow a clear and unambiguous specification or format, to allow for parsing.
Examples :

E-mail header: The text (body) is preceded by header lines indicating sender, recipient, subject, sending time stamp, receiving time stamps of all intermediate and the final mail transfer agents, and much more. See RFC 5322 for details. Similar headers are used in Usenet (NNTP) messages, and HTTP headers.

In a data packet sent via the Internet, the data (payload) are preceded by header information such as the sender's and the recipient's IP addresses, the protocol governing the format of the payload and several other formats. The header's format is specified in the Internet Protocol.

In data packets sent by wireless communication, and in sectors of data stored on magnetic media, typically the header begins with a syncword to allow the receiver to adapt to analog amplitude and speed variations and for frame synchronization.

In graphics file formats, the header might give information about an image's size, resolution, number of colours, and the like.
Source :
http://whatis.techtarget.com/definition/header
http://www.brainhost.com/help/website-development/what-is-my-website-header/
http://en.wikipedia.org/