Why a <cite> when I have <em>

The <cite> html element is supposed to convey more semantic information than just using an <em>. But what is the core difference in usage and pu开发者_StackOverflow中文版rpose of them?

<cite> goes around citations. <em> goes around emphasised text. They have totally different semantic meanings to things like search engines and screen readers.

It's easy to imagine a situation in which you want to typeset citations differently from emphasized text. This can be done easily using for instance style sheets thanks to the fact that the tags are different.