This paragraph illustrates two main kinds of emphasis. This sentence uses the em tag. By default, emphasis is italic. This sentence uses strong emphasis. The default formatting of strong emphasis is bold.
Of course you can change the formatting with CSS. This is a great example of semantic formatting. Rather than indicating the formatting of some text, you indicate how much it is emphasized.
This way, you can go back and change things, like adding color to emphasized text without the formatting commands muddying your actual text.