I just learned about something that Yahoo uses called "Web beacons",
which are created for anybody who is a member of Yahoo Groups.
Basically, these are cookies created by Yahoo, but that can be used by
Yahoo's (unnamed) partners. For example, if company X is a partner of
Yahoo, these cookies can be read from company X's Web site. (The beacons
can even be embedded in HTML-based email, so that companies can
determine when a certain email was opened.) The data is then apparently
reported back to Yahoo, who then aggregates it.
If you don't want Yahoo to track your movements around the Web using Web
beacons, you can opt out from the Yahoo Web site. Please note that this
must be done on a per-browser basis, not a per-basis. So if you, for
example, use both IE and Firefox, you will need to opt out for each browser.
For general information about Web beacons, see, for example:
For Yahoo's policy about Web beacons and to opt out (if you want to), see:
(Note that the page displayed after opting out is confusing. It has a
button labeled "Cancel Opt-out". Don't click this, as it cancels opting
out -- in other words, it opts you back in and you get Web beacons again.)