If a user clicks on the ad, the action triggers a post to their wall and sends an email to provide details of the promotional offer, which also gives the user an opportunity to share the offer with their friends.
This new advertising product is a study by Facebook to remain contemporary with other deal advertising efforts of social sites, local sites and deal websites. The goal is certainly an attempt to take market share away from Groupon and LivingSocial, as well as hedge off contenders such as Google before the search giant can firmly sink it's teeth in the market using Plus.