Susan Kish :
"Our emerging future will include three separate kinds of “worlds” – the Real World, the Digital World (2D Web, Internet), and the Virtual World (3D Web).
Under the umbrella of Virtual Worlds are emerging universes, ranging from MMORPGs (massive multiplayer online games, such as World of Warcraft), and Metaverses (Virtual Worlds that are primarily social vs. game oriented, such as Second Life), to MMOLEs (focused on learning and training environments), to Intraverses (putting up a virtual world inside the corporate firewall), to Paraverses (often also called Mirror Worlds, such as Google Earth).
Virtual Worlds are fundamentally immersive, visually compelling and highly social experiences. Trusted relationships, both personal and professional, emerge quickly in these environments and can carry over into the real world. Skish_virtualidentities These worlds provide a degree of control to individuals – control over their age, their gender, their appearance and their setting – that enables escape and fantasy. Pure curiosity, and the desire to explore and to create, causes many to visit – and often stay." (http://www.lunchoverip.com/2007/10/second-life-vir.html)
Status of Research on Virtual Worlds
"Virtual worlds are computer-mediated interactive environments in which human actors control one or more characters or avatars, software agents meant to represent them, in persistent-state but synchronous software environments where changes to the environment or the agent are permanently recorded and recalled from session to session. Virtual worlds have a strongly temporal character differing from other computer-mediated entertainment or simulation environments where there is no persistent record of interactions from session to session.
As a specific form of new media, virtual worlds have their origins in academic and applied computer science, most centrally in the early development of MUDs (multi-user dungeons). The first MUD was created in 1978 by a British undergraduate, Roy Trubshaw, and elaborated upon by Richard Bartle and Nigel Roberts.  Bartle went on to become a key early progenitor of both the phenomenon of virtual worlds and one of the key academic proponents of the scholarly study of virtual worlds. In fact, for many MUD designers, their worlds were both an expressive media form and an instrument for conducting systematic research in social psychology and social science.
During the 1980s, MUDs were made available to their users largely free of charge, maintained within university or research-institute facilities, experienced by relatively small numbers of people. Most were entirely “text-based”, meaning that the environment was represented almost entirely by words (occasionally some MUDs used letters to create simple ASCII-style graphics like maps) and action within the world was enacted with the use of a text parser. Many of these early worlds and virtual communities were designed to test either specific applications, such as LambdaMOO, which XeroxPARC research Pavel Curtis hoped could be a test bed for text-based synchronous conferencing systems  , or were designed by academic researchers as thought-experiments of a sort, to somewhat playfully test particular ideas about social or psychological behaviors. Over time, the number of MUDs grew and increasingly large numbers were designed and maintained by hobbyists or as a new media form that served creative and cultural purposes. Many of them, in either their academic or aesthetic manifestation, drew significantly on a genre of non-computer mediated games which slightly preceded the advent of MUDs, such as Dungeons and Dragons, which in turn drew on postwar fantasy literature, particularly Lord of the Rings. As the numbers of MUDs grew, the variety of underlying structures in their persistent worlds also multiplied: virtual worlds built around killing monsters and accumulating resources, virtual worlds built around social relationships, virtual worlds built around collective storytelling
In the mid-1990s, the first major pay-to-play virtual worlds began to appear. Some were text-based, but starting in 1996, commercially run graphical virtual worlds began to appear, with a number of them becoming substantially profitable. Everquest at its peak had as many as 500,000 subscribers in the United States and Europe. Several games in East Asia, particularly Lineage, have garnered very large numbers of users, though the nature of the local market makes the numbers very difficult to compare to those of other MMOGs. Most recently, World of Warcraft has achieved unprecedented global success and popularity, far outstripping Everquest at its peak.
The growth of commercially successful virtual worlds has spurred existing scholarly interest in the study of virtual worlds and diversified the range of disciplines involved in this research. Economists, psychologists, political scientists, ethnographers and legal scholars are now active in the field, not to mention scholars who define themselves as working within nascent disciplines of game studies or ludology. Unlike the scholars working on artificial societies and simulations, these scholars are studying a social and cultural phenomenon that is exterior to their own efforts: commercial virtual worlds are largely not a research-driven attempt to model social reality. However, one of the interesting aspects of scholarship on virtual worlds is that it often includes or involves some of the key designers or practicioners within scholarly debate, and the tradition of direct scholarly participation in virtual world design also remains strong even if the requirements for producing full-scale graphical virtual worlds are now well beyond the means of even the best-funded scholars (they cost many millions of dollars to design and maintain).
The chief interests of scholars working on virtual worlds largely divide into three major areas:
a) The internal economies of virtual worlds and their interface with real-world economic value. Most of the major commercial worlds have a substantial internal economies and the gameplay they offer customers is centrally driven by accumulative dynamics. Players invest labor to become more powerful in order to extract resources more effectively from monsters and the virtual world itself, which allows them to become more powerful and therefore extract resources more effectively still, and so on—a dynamic that some players appropriately call “the treadmill”. Much of the wealth created by this virtual labor is internal to the player’s avatar and cannot be abstracted from it, but in many virtual worlds, the player’s power is also amplified through the acquisition of virtual items which can be traded between players. A substantial secondary market on eBay and other auction sites has grown in which players sell off both virtual items and the avatars themselves for considerable sums. The economist Edward Castronova has become the key scholarly figure in the study of both the internal economies of virtual worlds and their connections to real-world economies, and was the first to rigorously quantify the “exchange rate” between value in one of these worlds and value outside of it.
b) The psychological questions raised by the relationship between players and their avatars: are avatars expressive of psychology of their human controllers, and in what ways? Do relations between avatars or the action of gameplay have a psychological impact on real-world players? Do virtual worlds create a novel context for psychological expressiveness? What difference does the visual interface make in the psychological expressiveness or consequences that follow on participation in a virtual world? (Some virtual worlds use an isometric 3rd-person perspective, while others use a 1st-person perspective, and still others allow players to switch between differing orientations.) What do formations and practices of identity in virtual worlds tell us about the history and practice of identity-forms in the real world? This field is especially dominated by questions about gender and sexual play in virtual worlds, but also struggles with a major evidentiary problem: comprehensive data about the demographics and social identity of players is closely guarded by the owners of major virtual worlds, and fine-grained studies of individual psychology and behavior are also made difficult by concerns about anonymity and typicality.
c) Questions about the social, political and legal structures governing virtual worlds and their evolution over time. Here there are both empirical questions about the social structures within games—what they are, how they came to be, how they change within a given virtual world and between virtual worlds, but also even more pressingly, questions about the relationship between real-world social and political practices and structures and the virtual world. Is a virtual world a way to compactly examine the particular existing character of contemporary societies (or some social subset of them) or is it a way to isolate and simplify some fairly universal dynamics governing human sociality and politics? Is it a model or a mirror?" (http://weblogs.swarthmore.edu/burke/?page_id=31)
Full introduction at http://skish.typepad.com/susan_kish/secondlife/SKish_VW-SL_sept07.pdf