Hi Sheryl,
Welcome to the Windows 7 Community!
I understand that you would like to know the difference between MHT file and HTM file. I am glad to help you.
MHTML, short for MIME HTML, is a web page archive format used to combine resources that are typically represented by external links (such as images, Flash animations, Java applets, audio files) together with HTML code into a single file. 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 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 standard, circulated in a revised edition in 1999 as RFC 2557.
HTML Web page used by Web browsers; contains markup code that is stored a plain text format; used to display and format text and images in a Web browser.
HTM files serve the exact same purpose as .HTML files. The ".htm" extension arose with the use of Microsoft Web development software, such as FrontPage, SharePoint Designer, and Expression Web.
HTM files are much less commonly seen than HTML files.
Hope it helps. For any further Windows related assistance, feel free to contact us and we will be happy to help.
Regards!