div is used in order to divide a page in to segments, meaning that if you put something in a div it will automatically add line breaks before and after it. Also, you can align a div, but you cannot align a span.
Wow, I never thought such a simple question can be so hard to answer.. Hmmm, how does one even explain this? Div, is like a big plate, while span, is like a small plate. Lmao! No, seriously though, div is almost like a table element without the TDs & TRs. While span is more of like a paragraph kind of thing, where it's more for phrases, & letters, sentences.
Well, div and span are usually used for styling with CSS and for DHTML code to pop up. The difference between div and span is that div makes your line break while span doesn't.