Automated extensions display information that shoppers find useful alongside your Product Listing Ads (PLAs) — such as free shipping costs or sale price figures. Per Google: "Promotional text, an optional message added to your ad groups in AdWords, was part of our first efforts to do just that. We noticed many of you use promotional text to highlight special offers or free shipping, so today, we’re excited to introduce new automated extensions for shopping ads (PLAs)."
Automated extensions use existing information you’ve provided in your Merchant Centre settings and data feed to display this info within your PLAs automatically. In the olden days, using promotional text ads, you had to manually enter the copy into AdWords.
Not only do these new extensions display in a digestible format for consumers and help differentiate your products, but they come at no additional work or cost for retailers. And the debut comes at an optimal time for retailers, as most are preparing to ramp up their holiday promotions soon.
As a big bonus, Google reported that automated extensions received better click-through-rates (CTRs) than promotional texts during testing.
Howver, Google has said "we’ll be retiring promotional text at the end of September 2015 and start surfacing automated extensions more frequently instead...For those who use promotional text, no additional action is needed. Your ads won’t be otherwise affected and will continue to reach shoppers searching on Google. If you want to highlight other offers aside from free shipping and sale prices, we strongly encourage you to take advantage of our other shopping extensions" (e.g. trusted stores, promotions & ratings).
See more from Google here: http://adwords.blogspot.co.uk/2015/08/new-automated-extension-for-shopping-ads.html