What are web cliques?
In the '90's when the internet was relatively young, the only way to have an online presence was to create one's own personal website. Those who did mainly saw webdesign as a hobby, with sites that ranged from blogs to fansites to collectives.
As the number of sites offering personal site hosting grew, so did the users, and as with any common interest, communities were eventually born out of a need to belong. Web cliques (or just "cliques") became a fun and free way for peronal site owners to connect with others of a similar interest. Each clique was structured around a particular theme (usually entertainment-, design-, or fandom-related), with its own set of rules and codes, which were often placed on a site's splash page (or front page).
The difference between a clique and other joinables—such as the fanlistings, which were popular in the early 2000's—is that members must have websites. In order to be listed, all members must first put up codes linking back to the clique page on their sites before joining. Cliques, therefore, are another way to promote one's site and gain more visitors.