Have you ever come across a url containing a reference to gclid, like this, in your weblogs:
and didn’t know where it was coming from, or what it was doing?
Well, search no more!
The gclid parameter is used by Google, and it’s value is used to link your Google Adwords campaigns to your Google Analytics account.
It’s in use since 2004, with the introduction of the URL Auto-tagging (which is on by default on your Google Adwords account), and is completely harmless.