UTM tags allow you to analyse your traffic more clearly. For example, you have 2 sources of your traffic: Facebook group and email database.
First UTM tags that you need -
utm_source and possible values are
The second valuable tag is
utm_campaign - allows you to define analytics for different campaigns that you make on the same traffic source. For example, you do 2 direct email campaigns per week. You can define in every campaign the
utm_source tag in your links like,
ad123, etc. It can be any value that will help you to remember what campaing it was.
All supported UTM tags are:
More information about UTM tags is here