A browser tells the site it is visiting about itself:
- the plugins it is using,
- the screen resolution it is set at,
- the fonts available for display,
- It’s preferred language/timezone
- and many other little details… (OS etc)
It does this in order for the remote site to be able to serve a properly formatted webpage or tailored content.
The problem is, that potentially the combination of the above mentioned features can mean that you can also be uniquely identified or be placed into a reasonably small group just by the information your browser passes to websites you visit.
Check this site to find out how unique your browsers finger print is: http://panopticlick.eff.org/
I connected with Ubuntu 10.04 and a Chromium browser and my data was unique despite them receiving 846,729 connections.
Just like your finger print or DNA you can be uniquely identified by your browser.