Google Analytics detects 3 default mediums without any customization: organic, referral, and none.
Organic: this represents traffic that comes from organic, or unpaid search results
Referral: any traffic that comes to your site from another site that is not a search engine
None: this medium is applied to users that came directly to your site by clicking on a bookmark or typing in your URLinto a browser. (Source: Direct/Medium: None)
Tracking social, email, and display campaigns in Google Analytics. Link tagging is adding extra information to the link that users click on to get to your site. This extra information that you provide is stored in campaign tags and overwrites the default categorization that would normally be assigned.
- Term (optional)
- Content (optional)
Link tagging is important in situations where you have multiple display ads. You want to know which ad is leading users to your site. One ad may have a tagged link and the other may not. The ad without a tag will not display campaign information and the one that has been tagged will. If you don't tag ads properly this will lead to incorrect data in your reports.
A helpful tool called the "URL Builder" can be found at support.google.com/analytics to help you build these campaign tags correctly.