(the advertiser, website, publication, etc. that transfers traffic to the owner, for example: google, facebook, bing)
(the advertising or marketing medium, e.g. cpc, banner, email)
(It is used to differentiate between similar content or links within the same ad. For example, if you have two call-to-action links in an email, you can use the utm_content variable to differentiate between them and determine which version is more effective.)