Each link in emails is automatically tagged with UTM tags. UTM tags are used by Google Analytics and many other software for reporting.
To enable/disable link tagging in your account, check the "Enable Google Analytics link tagging" checkbox on the Settings page under the Email Campaigns group.
Turning this setting ON ensures that links in email campaigns are tagged with UTM tags -- this behaviour can be overwritten for individual messages from the email editor sidebar.
When this setting is enabled, every link in every email campaign is tagged as follows:
utm_medium = email
value set to "email", cannot be changed
utm_campaign = [name of email campaign]
automatically populated with the name of your email, can be changed manually
utm_source = [list of recipients]
automatically populated with the name of your recipient list, can be changed manually
utm_keyword = [email subject line]
automatically populated with the subject line of your email, can be changed manually
utm_content = [blank]
not populated automatically, can be changed manually