Sitelinks are shortcuts to different pages on your website that appear below your ad. They allow you to show links to pages on your website in addition to your main landing page. Learn more in Google AdWords help article, Show your website links below your ad text.
We have found that 4-6 sitelinks work best with our clients.
Google has a policy that each sitelink in a campaign needs to link to a unique landing page on your site. As of fall 2012, Google will begin enforcing this policy. Read an article on what you need to know about Google’s sitelink policy update.