A web tease is a relatively new phenomena. It is something that only could have have developed with the technology we have now.
Generally a Web Tease follows the following format: using images that are available publicly, the author puts together a specific storyline. Usually in the first person, with a majority of photographs where the model is looking out at the viewer, a Web Tease gives instructions that make the user feel as though they are personally being instructed to follow through. While most are simply elaborate ways to masturbate and are fun in themselves, many also incorporate fetishes, bdsm, and other elements which introduce the viewer to new means to achieve sexual pleasure.
Viewers go through a series of pages, each one with new instructions (and usually a new photo). Following all the instructions doesn't necessarily lead to orgasm, which is part of the excitement of playing along. There is a special type of delicious frustration which comes from being aroused for 20-40 minutes and then denied release.