Google Remarketing ads are those ads that follow you around the web after you've visited a site running such a program. Creepy? Who cares. In such a distracted world, this is a great method for bringing visitors back to your site and reinforcing your brand message through repetition to a targeted audience.
Quickly, the way it works, is that you connect Adwords with Analytics (must be using Universal code). You then create "lists" in Analytics that could be as broad as "all visitors to my site", or narrowed down to "visitors who entered my shopping cart, but didn't purchase". Then in Adwords, you set up a display network campaign and create ads that only target these lists.
The reason this is effective is that here you're targeting a person who's already touched your brand. They've been to your site. They've sniffed the bait, but did they bite before leaving? Why did they leave? Maybe the phone rang. Maybe an Ello friend notification popped up. Consumers are bombarded by a million and one distractions each day and this is a great tool to refocus their attention back onto you.