It works like this: The site asks you to download a small applet containing the dialler. Once loaded on your system, the applet disconnects your ISP service and dials the pay-per-view site. You are then charged for however many minutes you remain connected to the site. The fact that you downloaded and installed the app is considered consent for the telephone charges.
The problem is that sometimes diallers are installed on your system without your consent, and change your dial-up settings to call expensive long-distance numbers. This scam is usually perpetrated by fraudulent companies that set up sites full of ActiveX multimedia content. Should you visit the site or click a pop-up ad for the site, the dialler would be downloaded onto your system along with the rest of the site's content. The only possible clue this has happened to you is the appearance of pop-up ads from sites you don't use; the real proof is the extra charges on your next phone bill.