Source (or traffic source) is the origin of your website traffic. It also includes traffic sources tracked via the utm_source parameter. For example, In the case of google / organic, 'google' is the traffic source In the case of bing / cpc, 'bing' is the traffic source In the case of tripadvisor / referral, 'tripadvisor' is the traffic source. In Google Analytics, 'source' names are case sensitive. So 'google', 'Google' and 'GOOGLE' are all treated as different traffic sources in Google Analytics.