No, it's not new. It's existed for a long time, and then a few years ago Google reinvented it with the word 'remarketing'. Basically it's an ad campaign that targets people who have already been to your site.
For example, suppose someone visits Hotels.com. A cookie is placed on their web browser to indicate they've been to hotels.com, and now, for the next few weeks, wherever they go on the web, someone is trying to sell them a trip because it's now known they're in the market to travel somewhere.
We use AdWords remarketing here on DaniWeb. Once you visit a section of the site (say, the C++ forum), we'll keep showing you ads wherever you happen to be on the web to invite you back to join our Software Development community.
Retargeting ads: It is a way for businesses to advertise to you after you visit their website or open their email.This method works by using banner ads and other methods of advertising to continually market a product or company to the potential customer in the hope that they will eventually make a purchase.
Retargeting is a technique which is used to display ads for the visitors who has already visited its website.For eg, if a person checks out some shoes in amazon and closes the website without any purchase then that person's cookies will be tracked and when that person visits some other website , then the shoe ads which he had been checking out in amazon will be displaying in another website.This is mainly done so as to increase the sales conversion of the website.