HTML Files Extensions - What are html files and how do I open a html file?

What are html files and how do I open a html file?

File Type Category File Format
.htm HTML Files Hypertext Markup Language File
.html HTML Files Hypertext Markup Language File
.htmlz HTML Files Zipped HTML eBook
.mht HTML Files MHTML Web Archive
.uhtml HTML Files UC Browser Saved Webpage
.vbhtml HTML Files ASP.NET Razor Web Page