[personal profile] dragonwolf 2009-05-21 02:42 pm (UTC)(link)
Span is actually supposed to be used in place of font, which is deprecated.

For example, <span style="font-size:.8em"> a section of somewhat smaller text.</span>

It's used to style a line-level (as opposed to its block level counterpart, <div>) section of an element, either through inline styles, or an entry in an external stylesheet.