If you’re working with a web tool that allows you to use HTML, you might find the cheat sheet below helpful.
I originally created it for students who are participating in a class blog run by WordPress. When they’re working on a new post, they can use the formatting toolbar in Visual mode to apply these option without needing to know the HTML code. But when they post comments on blog posts, they won’t have a formatting toolbar,so using HTML is the only way to apply formatting. But the codes will work anywhere that HTML is allowed.
(click on the thumbnail to see the full-sized version)