Control refers to the original version of your website where no change is made. You can determine the impact of your new versions in comparison to the control version of your website.
Variation is the changed version of the website that you want to test. You can create multiple variations of your website to determine what works best for your business.
The traffic on the campaign URL (specified under “Included URLs”) is equally distributed among all the versions. For example, if you have created a new variation of your existing webpage, then out of all the visitors landing on the campaign URL, 50% will see the Control (or the original webpage), and the rest 50% will be shown the Variation (or new version of the webpage). The traffic distribution is random and can be changed as explained in this article.
For a Split URL campaign, out of all the visitors landing on the control URL, 50% will stay on Control and the rest 50% will be randomly redirected to the Variation URL. That is, for a Split URL campaign, visitors become the part of the campaign only when they first land on the Control URL.