A header may refer to any of the following:
1. In general, when referring to data or data transmission, a header describes information placed in front of other data, known as the body or payload. The header describes the payload and information to help devices know how the data should be handled.
2. A header or heading is text at the top of a page in an electronic document or hard copy. For example, in Microsoft Word, a header could be created in a document to display the page number of each page. By contrast, a footer is at the bottom of a page in an electronic document or hard copy.
3. On a web page, the header or web page header is the top portion of a web page containing the company name and logos. Contained between opening and closing <header> tags, this section also often contains a navigation bar to help users move between common areas on the site.
4. An HTTP header is data transmitted as part of the HTTP (hypertext transfer protocol) network protocol. The header contains important data communicated between a browser and a web server, such as the origin and nature of the HTTP request.
5. When discussing the motherboard, a header can refer to pins on the motherboard that allow additional ports to be added to the computer. See our 1394 and USB header definition for further information.
6. More commonly known as an e-mail header, a header is data contained at the beginning of an electronic message containing information about its contents. The header helps identify the message, its source, destination, and other information required to decode and understand it. The technical specifications on e-mail headers are available in RFC821, RFC822, and RFC2045.
7. With a file, a header is information at the beginning of the file that gives additional details about the file. For example, with some image files, the header could contain the image's bit depth, dimensions, resolution, etc.