And are they supported by accessibile browsers (readers)?
I guess the question is pretty simple. Can I signal in my classnames that this is a search field, this is a box with popular tags, these are the main navigation links, this is a box with the latest comments, this is a link for your profile page, you can logout with this link, and so on...
The short answer is no. The longer answer is WAI-ARIA can do some of what you want but there's no guarantee that screen readers will support it. For a discussion of WAI-ARIA roles see http://www.w3.org/TR/wai-aria/roles Pay particular attention to section 5.4, things like banner and contentinfo may be useful to you. Do not depend on WAI-ARIA support though; make sure that you use good HTML including descriptive alt tags and headings to separate your document into appropriate sections.