Premium Camsites are paid webcam sites. You have to pay here for your membership / or per show and sometimes for some functions during use. Premium camsites usually have a preview, also known as a "Free Chat". As a visitor, you can first become acquainted.
An advantage of the paid cam site is that you usually watch a show one on one or with a (very) small group of people. This gives a model more time to be personal with you. It's a better and easier way to bond with your model. If you are a model yourself on a paid site, then you are guaranteed to be paid by the customer before you get naked. A disadvantage of these websites is that you usually don't actually see what you will get, before you pay.