Site links have been around for a long time now. Since 2006 in fact. You’ve probably noticed them, used them, and forgotten about them without ever knowing what they were called. And now they’ve been updated by Google.
Site links are the small secondary links that you get under a search engine result, when there are other pages of that site or domain which might contain the information that you’re searching for.
Original Site Links
When they were first launched, site links were small links appearing just under the description for the search result, and getting them displayed was often a question of whether you had “table of contents” with page anchors like Wikipedia pages do, in which case those anchor links would be displayed as your site links, to allow users to jump to a specific point on a page.
After a while, Site links started to be links to other internal pages that seemed relevant to a users search, and I’m guessing that they proved popular, because Google has revamped them considerably, as you’ll see.
Updated Site Links
Instead of the original tiny links, or even the later, slightly larger ones, Google has now made site links a very dominant part of results, if your site structure and navigation makes it possible for them to determine site links, and if Google thinks that site links will help people find what they’re looking for on your site.
Controlling Your Site Links
Although the site links are automatic, Google has given people the option to exclude certain pages from site links manually, through their webmaster tools.
According to the webmaster tools help for site links, (available here), it’s possible to indicate that a specific URL is not an appropriate site link for a specific page.
They don’t guarantee that it will never again be used as a site link, but they’ll take your opinion into account.
Getting Site Links
As Google explains, site links are determined automatically, and they depend on the structure of your site being such that Google’s algorithm can determine appropriate site links. You can demote site links as explained above, but you can’t create them yourself.
The best thing you can do is make sure that your URL and navigation structure is easily readable by Google, and that your pages contain unique and relevant information for your visitors. Site links can help your visitors find more relevant pages, and they can also increase your coverage of the search engine results page, especially now that they’re full size links.