Google introduces a new one-line format to get the benefits of AdWords "Ad Sitelinks" for even more of your campaigns, including your more generic, unbranded campaigns.
"we're also enabling option to set up Ad Sitelinks for any campaign. When a user's query matches a keyword in your Ad Sitelinks-enabled campaign, Google will automatically determine if your ad qualifies to show Ad Sitelinks and whether to show two-line or one-line format based on the quality of that ad. Ads that currently qualify for the two-line format willn't be affected by the addition of new one-line format," explains Google.
You should keep in mind that Ad Sitelinks is a campaign-level extension, so links you create should be relevant to any ad in that campaign. You'll find the option to set up Ad Sitelinks in your Campaign Settings tab. It'll appear as "Show additional links to my site" under "Ad extensions" section.
More Info: Ad Sitelinks FAQ