What is the GCLID parameter?

Gclids or (Google Click Identifier), are the globally unique tracking parameters used by Google to pass information back and forth between Google AdWords and Google Analytics.

If you enable URL auto-tagging in Google AdWords, Google will append a unique parameter(gclid) on your destination URLs at run-time. The gclid parameters will not be shown in your ads' on your paid ads, but they will redirect and append the gclid. Your server logs will also preserve the gclids as well.

The gclid parameter enables data sharing between Google Analytics and Google AdWords. For example, the "Traffic Sources" feature in Google Analytics to segment your campaigns based on ad text and other PPC-related campaign metrics and dimensions, relies on gclid.

From WordStream.com: "An unfortunate side effect of Google Adwords & Google Analytics integration is that it kills your search query data. Critical data pertaining to clicks and conversions are misattributed to the keyword in your AdWords account that triggered your ad, not the actual search query that was entered by the searcher."