In addition to what Mike wrote, they, like <div>s, can be used as hooks for style rules. In other words, if there is something which you wish to style, but there isn't any exsisting tag which includes exactly the right amout--no more and no less--you would use a <span> for inlines and a <div> for blocks.
Ever since I've been using the standards approach to web design, I've never used spans at all. What's the point of using them?
Mordechai
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