A long article by Vanessa Fox on Search Engine Land about this.

<<A new feature has appeared in the Site Configuration Settings Sections of Google Webmaster Tools. The setting, called Parameter Handling, enables site owners to specify up to 15 parameters that Google should ignore when crawling and indexing the site.

Google lists the parameters they’ve found in the URLs on your site, and indicates whether or not they think they those parameters are extraneous (with a suggested “Ignore” or “Don’t ignore”. You can confirm or reject those suggestions and can add parameters that aren’t listed.>>

http://searchengineland.com/google-lets-you-tell-them-which-url-parameters-to-ignore-25925