Control refers to the original version of your website where no change is made. The control is the base against which all your website test results are compared.
Variation is the modified version of the website that you want to test against Control. You can create multiple variations of your website to determine what works best for your business.
The traffic on the test URL (specified under “Included URLs”) is equally distributed among all the versions. For example, if you create a new variation of your existing webpage, then among all the visitors landing on the test URL, 50% will see the Control (or the original webpage) and the rest 50% will see the Variation (or new version of the webpage). The traffic distribution is random, and you can configure the allocation of traffic among the variations, as explained in this article.
For a Split URL test, among 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. Therefore, for a Split URL test, visitors become a part of the test only when they first land on the Control URL.