Wikipedia
MHTML, short for MIME HTML, is a web page archive format used to combine in a single document the HTML code and its companion resources that are otherwise represented by external links (such as images, Flash animations, Java applets, and audio files). The content of an MHTML file is encoded as if it were an HTML e-mail message, using the MIME type multipart/related.
The first part of the file is an e-mail header. The second part is normally encoded HTML. Subsequent parts are additional resources identified by their original URLs and encoded in base64. This format is sometimes referred to as MHT, after the suffix .mht given to such files by default when created by Microsoft Word, Internet Explorer, or Opera. MHTML is a proposed open standard, circulated in a revised edition in 1999 as RFC 2557.
The .mhtml (Web archive) and .eml (e-mail file) file extensions are interchangeable (the files can be renamed). The first can be sent by e-mail (and displayed by the email client if the html code is basic enough) and an e-mail message can be saved to an OS file and renamed to a Web archive extension.