Yahoo’s web beacons are a device that Yahoo uses to profile their customers and track user activity across multiple sites. They embed GIFs in various pages and messages to keep an eye on where their registered users go. This system is similar to those used by DoubleClick and HitBox, the images they deliver to you come with cookies that are associated with your profile. These systems can be used to violate a user’s privacy, especially the Yahoo system, but they’re not quite the same as Microsoft Passport, I don’t think.