Are you running a campaign and you want to know who came from which campaign? It is SUPER simple with the utm_source property. 

What is the idea?

When creating your URLs in your google/Facebook ads for example, add at the end 

?utm_source=facebook_ad1

Indicating in this example that it is our first ad in Facebook (of course you can add better names).

Now how to track that when someone registers or fills a form/checkout?

Go to your Contacts section (the CRM) -> Properties and create a property called utm_source we will define it as a property so we can see it for every user that comes (create it as a TEXT input)

Now all we have to do is add this property to our forms. Go in the form fields or the additional fields if it is a checkout or registration and add a Hidden Field with name being utm_source.

Then go in the settings of your field and map that field from the dropdown to the utm_source that we just created in the Contacts as a property.

That's all! Now when someone submits your forms and has the utm_source as a URL from which they came from, the value of the source will be recorded for the user and you will know where they came from.