Why UTM Parameters
UTM parameters are Google Analytics' way of assigning advertising from a non-Google source to the correct source, medium and campaign in Google Analytics. Without these parameters, advertising traffic will appear as Direct or Referral traffic. It could lead to data being misinterpreted and advertising campaigns being undervalued.
Some platforms, like Bing Ads, will have an option to add these parameters automatically while others, like Facebook Ads, will require you input them manually for each campaign.
Campaign URL Builder
Google provides a Campaign URL Builder to help generate URLs with correct UTM parameters. There are four required fields:
URL: the landing page for the ad.
Source: such as facebook (note, it's good practice to keep everything lowercase).
Medium: for social ads this is most commonly referred to as display traffic.
Campaign: a name for the campaign of your choosing.
For example, a summer promotional advertising campaign for Facebook might look like: https://www.example.com/product/?utm_source=facebook&utm_medium=display&utm_campaign=summer_promotion