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 its information to help devices know how the data should be handled.
2. A header 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 in the top corner 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 that contains 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 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 talking about 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 that contains information about its contents. The header is used to identify the message, its source, and destination, as well as other information that may be required to decode and understand it. The technical specifications on e-mail headers is also 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.