What's the difference between and , and ?

Answer 1

b or i means you want the text to be rendered as bold or italics. strong or em means you want the text to be rendered in a way that the user understands as "important". The default is to render strong as bold and em as italics, but some other cultures might use a different mapping.

Like strings in a program, b and i would be "hard coded" while strong and em would be "localized".