Ads.txt stands for "Authorized Digital Sellers". It is a mechanism on how a publisher/website owner can tell potential buyers that the traffic they are buying is valid. It also tells the buyer that the SSP/DSP from which they are receiving the traffic is wanted/allowed by the publisher. This prevents third parties ("ad-frauders") to send traffic to buyers and claim that the traffic originates from your website (by falsifying the referrer domain in the bid-request). Read more
Ads.cert is a standard which is designed to sign and verify messages send via OpenRTB (RTB-Requests), allowing a seller to prove that the sold inventory is valid. A message is signed using private keys and can be verified using public keys. This way a buyer can ensure that the offered inventory is valid and adfraud can be reduced. Read more
App-ads.txt is the ads.txt standard for mobile apps. It allows buyers to verify that a seller is really allowed to sell inventory for this certain app. It combines the logic from ads.txt with meta information from App stores.