In information technology, Header refers to supplemental data placed at the beginning of a block of data being stored or transmitted, which contain information for the handling of the data block.

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

• In an e-mail, 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 822 for details.)

• In a data packet sent through 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 graphics file formats, the header might give information about an image's size, resolution, number of colors, and the like.

• In some programming languages (for example C and C++) the functions are declared in header files.