ARIA landmarks are used on pages to structure the page for the blind user. Think of them as an outline for the page. The JAWS user is able to bring up a landmarks dialog, which acts as a table of contents, from which s/he can jump to any point in the page. That sure beats having to reading from left to right, top to bottom to find something s/he is looking for!
To pass accessibility requirements, everything in a page must be contained within an ARIA landmark (i.e. no orphaned content, please). For your convenience, I have listed them below. You will note that there is a generic "region" role which you can use to mark any part of the page that doesn't fall into one of the landmark categories. It isn't technically a landmark role, although I have included it in the list because it can be used when one of the more specific landmark roles doesn't fit.
As a developer, you'll need to add these roles to the relevant html tags on your page. As a designer, you should indicate to developers how you want your page to be structured.
The List of LandmarksClick on the landmark to be brought to the official description of it in the ARIA spec.
- application/document - we're starting off with one that is mostly for developers. These roles trigger how the screen reader intercepts keyboard events. The first is for areas of the page that behave like an application (which could be the whole page in some cases) and the second is for areas of the page that are read like a document (an email, for instance). (Technically, document is not a landmark role, but it is related to application, so it is listed here for completeness.)
- banner - a banner is just what you think it is. It contains site-oriented rather than page-specific content.
- complementary - this area contains content relative to the main content (and often at the same level) but the content can also stand on it's own.
- contentinfo - think of contentinfo as a footer containing info about the page (a legal statement rather than additional navigational links). There should be only one per any application/document region.
- form - we all know what a form is. However, the one gotcha is that search has it's own defined landmark, so use search instead of the more generic form in that case.
- main - this contains the main content of the page (actually, technically, of the application or document, one per) and is used as an alternative to "skip to main content" links.
- navigation - a collection of navigational elements (usually links) for navigating the document or related documents. (This description is straight from the ARIA spec.)
- search - an area of controls that performs a search. Not just the text input field, but any search modifying fields and submission controls. (e.g. scope dropdown + search input field + submit search button)
- region - a large perceivable section of a web page or document, that is important enough to be included in a page summary or table of contents. (not technically a landmark, but it is a fallback role if there are areas of the page that don't fit into any of the other landmarks)
In most cases you need to specify a label along with the landmark to help the user determine its purpose. The ARIA spec contains the labeling requirements for the individual landmarks.