A web reader is simply a web / browser based means to view newsgroups and there associated content. Typically offered as an add-on service to existing subscriptions. For the casual user, web readers can be great as they allow for access no matter where you are, assuming you have access to a web browser.
Webreaders typically allow for the queuing, downloading, extracting and viewing of usenet material.
Some have even been modified to work on cell phone browsers. Typically allowing users to view photos and video files.