Apple announced plans for the ads back in June and developers have participated in a beta testing program since that time. The App Store Search Ads system finally went live on September 28, inviting developers to purchase ad spots through the program. Those ads are now viewable by the public as of this morning.
With Search Ads, developers are able to bid to have their app listed as the top result when certain keywords are used, in order to improve app discoverability. Ads are available through an auction system, with one ad displayed to customers per search result page.
Ads are clearly marked as such and the content they show is the same as that on the App Store app description pages, since they use App Store metadata and imagery.
According to Apple's Phil Schiller, Apple "thought about how to carefully" do search ads in a way that "customers will be happy with." Apple also believes the ad auction system is fair to developers, including indie developers.
As of last week, developers who signed up to offer Search Ads have been given a free $100 credit to get started with the program.