The section element is one of the new HTML5 elements, like header, footer, and other semantics. As far as I know, they work quite the same as a div, only the div is a more generic way of dividing content. The section is used for a specific group of content.
If your co-developer is using section everywhere, he is using it wrong and he has to use the div element. It is possible to use more section-elements.
With the use of section, the developer has to call it only once in the CSS-file with 'section{}'. Kinda like a class attribute, but used for specific areas on the web page.