The Discover miles work as following: you buy your ticket like you normally would and then, when the ticket purchase appears on your statement, you apply your Discover miles to that purchase on your statement. The redemption ratio is 100 miles = $1. You don’t have to pay off that credit card purchase once you redeem miles for it. The miles you earn with Discover it Miles card are nontransferable, meaning you won’t be able to transfer them on to another airline credit card or frequent flyer program. So, basically Discover miles are cash back rewards.