Ad extensions are a type of Ad Format and they show extra information about your business - hence the name extension. Some of the extensions can be added manually and some are automatic - the aim is to make your advert help users more, stand out better, provide better click rates- in that order.
There is no additional cost to extensions, however you will be charged for clicks on some interactions on the extension. For example clicks on a call button, a download button, (for app extensions), and map directions all cost the normal click cost. The free clicks are seller ratings, reviews, and social annotations.
Part of the automated system is highlighting the first description line where it can sometimes be added to the headline. It will only generally use this line if it is a complete phrase, run on lines are ignored. This results in a more prominent headline and hopefully higher click through rates and therefore lower costs.