Zorking is a variant on railroading. In traditional railroading, the PCs are limited to take only certain actions, with other actions being blocked, negated, or ignored. In zorking, however, the PCs can proceed in a number of ways, but most of those paths are futile. The GM is looking for a specific course of action, and only that specific action will allow the adventure to progress. In extreme forms, the GM does not even attempt to shape the PCs actions, instead repeatedly insisting that "nothing happens."
Zorking is symptomatic of a lack of flexibility, a weakly developed setting, and a single-minded determination to tell a story, while at the same time providing the general form of a role-playing game. The GM is passive, waiting for the PCs to act, rather than simulating an interactive and dynamic environment in which exciting things happen. They may also expect players to read their minds, or worse, actually enjoy the frustration of the stymied players, seeing it as a form of "challenge."
Zorking comes from the computer game Zork. In older text-based games with limited language parsing, players frequently had to type very specific instructions, often unintuitive ones, in order to solve a puzzle. Similarly, posters on RPGnet use the term pixelbitching by way of comparison to older computer games where you had to successfully click on small, inobvious graphical features, sometimes just a few pixels wide, in order to solve a puzzle.