<section> means that the content inside is grouped (i.e. relates to a single theme), and should appear as an entry in an outline of the page.
<div>, on the other hand, does not convey any meaning, aside from any found in its class, lang and title attributes.
So no: using a <div> does not define a section in HTML.
From the spec:
The <section> element represents a generic section of a document or application. A section, in this context, is a thematic grouping of content. Each section should be identified, typically by including a heading (h1-h6 element) as a child of the <section> element.
Examples of sections would be chapters, the various tabbed pages in a tabbed dialog box, or the numbered sections of a thesis. A Web site’s home page could be split into sections for an introduction, news items, and contact information.
The <section> element is not a generic container element. When an element is needed only for styling purposes or as a convenience for scripting, authors are encouraged to use the <div> element instead. A general rule is that the <section> element is appropriate only if the element’s contents would be listed explicitly in the document’s outline.
The <div> element has no special meaning at all. It represents its children. It can be used with the class, lang, and title attributes to mark up semantics common to a group of consecutive elements.
Note: Authors are strongly encouraged to view the <div> element as an element of last resort, for when no other element is suitable. Use of more appropriate elements instead of the <div> element leads to better accessibility for readers and easier maintainability for authors.
<div>: The HTML element (or HTML Document Division Element) is the generic container for flow content, which does not inherently represent anything. It can be used to group elements for styling purposes (using the class or id attributes), or because they share attribute values, such as lang. It should be used only when no other semantic element (such as <article> or <nav>) is appropriate.
<section>: The HTML Section element (<section>) represents a generic section of a document, i.e., a thematic grouping of content, typically with a heading.
The numbers in the table specifies the first browser version that fully supports the element.
In that vein, a div is relevant only from a pure CSS or DOM perspective, whereas a section is relevant also for semantics and, in a near future, for indexing by search engines.
Just an observation - haven't found any documentation corroborating this
If a section contains another section, a h1-header in the inner section is displayed in a smaller font than a h1- header in outer section.
When using div instead of section the inner div h1-header is diplayed as h1.
some more text
-- the Level2 - header is displayed in a smaller font than the Level1 - header.
When using css to color h1 header, the inner h1 were also colored (behaves as regular h1).
It's the same behaviour in Firefox 18, IE 10 and Chrome 28.
Take caution not to overuse the section tag as a replacement for a div element. A section tag should define a significant region within the context of the body. Semantically, HTML5 encourages us to define our document as follows:
This strategy allows web robots and automated screen readers to better understand the flow of your content. This markup clearly defines where your major page content is contained. Of course, headers and footers are often common across hundreds if not thousands of pages within a website. The section tag should be limited to explain where the unique content is contained. Within the section tag, we should then continue to markup and control the content with HTML tags which are lower in the hierarchy, like h1, div, span, etc.
In most simple pages, there should only be a single section tag, not multiple ones. Please also consider also that there are other interesting HTML5 tags which are similar to section. Consider using article, summary, aside and others within your document flow. As you can see, these tags further enhance our ability to define the major regions of the HTML document.