Try to move your mouse's cursor over one of this page's paragraphs, you will see it changing right away.
You will not see any change if, unfortunately, you are using an old browser (Internet Explorer not to tell).
You should not stick to the examples I give you here, but feel free to let your imagination take over.
You want to set paragraphs to become bold when your mouse touches them? Great, you know how to proceed.
You want to set paragraphs with a background image when your mouse touches them? This is also something you know how to do now!
One last thing: I only did it for paragraphs here, but :hover will also work with titles, <em>, <strong> etc.