Div is a block level element. Span is inline. Examples provided at this site: http://www.learnwebdesignonline.com/htmlcourse/span-div.htm
In simpler words, DIV starts from a new line while SPAN does not. <span>Sweet</span> <span>Home</span> will display: Sweet Home while <div>Sweet</div> <div>Home</div> will display: Sweet Home - Cybernaut
Also note, this behavior is by default only and you can change this behavior using styles: <span style="display:block;"> </span> will act as a div (block) <div style="display:inline;"> </div> will act as a span (inline) Cybernaut.
But you really shouldn't. That's why they're separate elements. I can't imagine any good reasons to reverse this behaviour