Wikipedia
AdChoices is a self-regulatory program for online internet-based advertising that exists in the United States, Canada and across Europe. The program calls for advertising companies to establish and enforce responsible privacy practices for interest-based advertising, aimed to give consumers enhanced transparency and control. Companies adhere to a set of principles that are enforced by accountability programs.
"Interest-based advertising" (also known as "online behavioural advertising" or " behavioral targeting") selectively displays ads based on browsing history, primarily using cookies, to users most likely to identify with and respond to the ad's specific content. The AdChoices icon is shown automatically by companies part of the self-regulatory program, and is meant to indicate to consumers when interest-based advertising data is being collected or used. By clicking on the icon (which is usually found in the top right corner of an online advertisement), a consumer can learn more about the ad or a website's collection practices, and be provided with the ability to opt-out of such targeting.
Although it is possible to opt out of interest-based advertising through the AdChoices program(s), opting out does not block advertisements nor prevent tracking of web browsing history by use of other ways (e.g. Flash cookies). The US and Canadian AdChoices programs require that participating companies not use Flash cookies or similar locally shared objects for online interest-based advertising purposes.