An Adult Verification System or AVS is a computing system used by a website to confirm that the user attempting to access their website is of the age required (usually by law) to view the website's content, which could include sex, nudity, violence or bad language. The system is used to legally protect companies in regards to laws against disseminating pornography to a minor.
These systems always use a credit card. The Adult Verification System is usually provided by a third-party company. In that way the same company can provide adult verification for multiple websites, thus becoming more user friendly.
In many cases the use of an AVS is largely a method of making a site into a pay site without actually calling it one — the fees charged by AVS systems are significantly more than enough to prove access to a credit card, and many of them rebill on a regular basis. And of course, minors can get access to credit cards in various ways. AVS systems often advertise the number and "quality" of sites that subscription to their services allows. Whether the use of an AVS would protect a site from liability if a minor actually accessed the site has not yet been tested in court.