A "purity test" is intended to measure the user's percentage of "purity", or non-exposure, to a certain field. The name derives from virginity tests which have historically been practiced in many sexually restrictive cultures and are still practiced in some nations today. The oldest and most common purity tests are intended to measure sexual purity. There are hundreds of different purity tests on nearly every possible subject imaginable, from sex to computers to Gilligan's Island. Purity tests are closely related to online personality tests.
Most purity tests have possible scores anywhere from 0 to 100%, but most scores in well-designed tests are designed to fall between 40-60% pure/corrupt (standard distribution curve). Purity tests ask numerous (sometimes embarrassing) questions of their users, and as they say, all technicalities count. These tests have anywhere from 50 to 1500 questions.
Extremely popular among people aged 13-40, purity tests are the subject of much discussion and quite a few parties. It is common to post purity test scores, discuss them with friends, and even plan to 'improve' them, using the purity test itself as a 'checklist' of things to do/try. They are easily obtained using any search engine (if you can find this article, you can find a purity test).
Purity tests are also a quick way to get anonymous information about the denizens of the Internet. One purity test is actually an AI experiment attempting to identify the test-taker's gender by the answers given to certain questions.
External links
The Armory (http://www.armory.com/tests)
The Unisex, Omnisexual Purity Test (http://www.nmt.edu/~kscott/purity/)