When using Adwords, Google will automatically add Auto-Tagging to your URLs if you do not customize your options. This will cause all your links ending in ".html" to fail when google adds their tags to them. This article will explain this in-depth and provide instructions on how to change these settings to have your links from google work properly.
For Google Analytics to display details about your AdWords keywords and costs, you must do one of the following:
Use Destination URL auto-tagging, or
Manually tag your keyword destination URLs with tracking variables.
Auto-tagging appends a unique ID to the end of the destination URL. This unique ID allows Analytics to report the click details: which AdWords keywords brought a visitor to your site, which campaign that keyword was from, and how much that click cost your account. You can calculate return on investment (ROI) by associating this data with goals or e-commerce conversions.
The parameter used in auto-tagging is called "gclid" and will show up in your landing page URL when a user arrives at your site from your ad. For example, if your site is www.mysite.com, when a user clicks on your ad, it will appear in the address bar as:
Auto-tagging is selected by default when linking your AdWords and Analytics accounts. You can manually enable or disable auto-tagging in AdWords.
To enable/disable auto-tagging:
Sign in to your AdWords account.
Click the My Account tab and select Account Preferences.
In the Analytics section, click Edit.
Modify the Destination URL Auto-tagging checkbox.
Click Save changes.
To determine if auto-tagging will work for your website:
Append ?glicd=test or &gclid=test (if your destination URL already contains other parameters) to the end of your AdWords destination URL
Paste this modified URL into your browser's address bar, and press Enter
Auto-tagging will not generate an error if the resulting page displays gclid=test in the address bar.
If the resulting page does not contain gclid=test, please notify your webmaster and refer to google's troubleshooting guide (in English only).
If you would rather track your clicks manually, please read How do I tag my links?