What is Yahoo's GCID

[flashfan]

When click a link in SERP, it returns http://www.example.com/?GCID=C15620x005-tld. What is the GCID?

[edited by: martinibuster at 9:36 pm (utc) on Feb. 21, 2006]
[edit reason] Examplified URL. ;) [/edit]