TXT is a file format that contains the text, organized in rows. Text files opposed binaries that contain data that is not intended for interpretation as a text (encoded sound or image). The text file can contain both formatted and unformatted text. Since the very simple text files, they are frequently used for storing data.
HTML is a Web format file. HTML source code can be changed in a text editor. HTML files are being developed for future use in the users web browser, allowing you to format text, images and other materials required sites. File with this format use tags to build web pages. HTML code is parsed Web browser and usually not visible to the user.