How do web beacons work?

[Lance Spitzner <lance () honeynet org>]

Okay, what exactly is a web beacon?  Its sounds kinda like a 
honeytoken, but I've never heard of the term before.  Now that I did a 
google, looks like quite a few are using them to track users.  What 
exactly is a 'web beacon' and how does it technically work?

  http://privacy.yahoo.com/privacy/us/beacons/details.html

Thanks!

lance