"Div" HTML tags instruct a section of code to use a particular style. These instructions always include a paragraph break. "Span" tags also instruct an HTML section to use a certain style, but they do not instruct any formatting on their own.
A span tag simply applies whatever style rules are within it to the block of content. Therefore, span tags do not include paragraph breaks. A web designer who works with Cascading Style Sheets may use these two tags interchangeably. The most common use of these tags is to separate content on a page. The more content on a page, the more important these tags become.