Use the URL Querystring retargeting method to cookie users based on the querystrings that appear in the pages they visit. URL querystrings are the words you see in a URL after a "?". For example, http://domain.com/?querystring=true. In that example, everything after the "?" is part of a querystring. Here are some examples:
Scenario: You want to create an Audience that captures people who sign up for your app. After signing up, a user lands on a page with a URL like this: domain.com/?sign_up=1
Solution: Create a URL Querystring Audience with "sign_up=1" as the value. This will cookie anyone who reaches a page with that phrase or string in the URL after a question mark.
Scenario: You want to create an Audience of people who came to your site from Adwords. You are using a utm_campaign parameter of "adwords" in the click URLs for your Adwords campaigns.
Solution: Create a URL Querystring Audience with "adwords" as the value. This will cookie anyone who comes to a page with "adwords" after the question mark in the URL, which should be only people coming from your Adwords campaigns.