By default, Google Analytics tracks all referrals that send traffic to your website. For example, every time a user is referred to your site from Bing, Google Analytics automatically captures the search keyword, the source (e.g. bing.com) and the medium (organic).
Google Analytics also allows pushing campaign-identifying information by adding tags to the end of the marketing destination URLs, in addition to out-of-the-box traffic sources information that are tracked automatically.
Tags are variables that contain campaign information. For example, ‘utm_medium=social’ identifies a social media campaign and ‘utm_campaign=mothers-day-2012’ identifies the Mother’s Day campaign for 2012.
Below you will find a few articles and resources to help you further understand campaign tagging in Google Analytics.
Articles: