714
<div> is a block-level element and <span> is an inline element.
If you wanted to do something with some inline text, <span> is the way to go since it will not introduce line breaks that a <div> would.
As noted by others, there are some semantics implied with each of these, most significantly the fact that a <div> implies a logical division in the document, akin to maybe a section of a document or something, a la:
<div id="Chapter1">
<p>Lorem ipsum dolor sit amet, <span id="SomeSpecialText1">consectetuer adipiscing</span> elit. Duis congue vehicula purus.</p>
<p>Nam <span id="SomeSpecialText2">eget magna nec</span> sapien fringilla euismod. Donec hendrerit.</p>
</div>