Source: Every referral to a web site has an origin or source. Possible sources include: “google” (the name of a search engine), “facebook.com” (the name of a referring site), “spring_newsletter” (the name of one of your newsletters), and “direct” (users that typed your URL directly into their browser, or who had bookmarked your site).
Medium: Every referral to a website also has one. They include “organic” (unpaid search), “CPC” (cost per click, i.e. paid search), “referral” (referral), “email” (the name of custom ones you have created), “none” (direct traffic has a medium of “none”).
Keyword: When SSL search is employed, the Keyword will have the value (not provided).
The campaign is the name of the referring Google Ads campaign or a custom campaign that you have created.
Content identifies a specific link or content item in a custom campaign. For example, if you have two call-to-action links within the same email message, you can use different Content values to differentiate them so that you can tell which version is most effective.
You can use Custom Campaigns to tag links to use your own custom values for Campaign, Medium, Source, and Keyword.