Virtual Worlds (sometimes referred to as Virtual Environments) are interactive and/or collaborative representations or simulations of real-world or fictional objects, events and characters. Virtual Worlds are most typically networked (i.e. connect users via the internet) to enable real-time communication between players.
Simulation is a related concept whereby an imitation of the operation of a real-world process or system occurs over time. Often, a simulation is run in a Virtual World or Virtual Environment, but it could also be carried out in a real-world Laboratory, "set" (as in TV set) or other simulated physical environment. The act of simulating something first requires that a model be developed; this model represents the key characteristics or behaviors/functions of the selected physical or abstract system or process. The model represents the system itself, whereas the simulation represents the operation of the system over time.
Simulation is used in many contexts, such as simulation of technology for performance optimization, safety engineering, testing, training, education, and video games. Often, computer experiments are used to study simulation models. Simulation is also used with scientific modeling of natural systems or human systems to gain insight into their functioning. Simulation can be used to show the eventual real effects of alternative conditions and courses of action. Simulation is also used when the real system cannot be engaged, because it may not be accessible, or it may be dangerous or unacceptable to engage, or it is being designed but not yet built, or it may simply not exist at all as of yet.
Simulations .vs. Virtual Worlds
In Simulations only one person is interacting with the simulation at a time. You encounter a virtual character like a doctor and he or she responds to you based on a pre-programmed script. In physical Simulations, such "pre-programmed scripts" may be textual lines or roles to be fulfilled by actors (other people), but those actors may not interfere with the simulation or stray from the script. While a group may work together to help make decisions whilst observing the learner navigating through the simulation, the learner is only interacting online with the program (or programmers) behind the simulation.
In a true Virtual World, multiple learners are “inside the simulation” at the same time. You can either interact with the pre-programmed items in the world or interact with other learners or instructors who are also in that world and actively engaging with it and each other. The online world becomes the environment in which learners interact and respond to one another. In physical Simulations you could approximate a Virtual World through a combination of placed items or forced circumstances along with adhoc/unscripted interactions, similar to improvisation (improve theatre, free wordplay, supervised real patient visitations, etc). 
Role-Playing Games (commonly abbreviated RPGs) are story-driven, character-centric games that are typically (but not always) less focused on action and more focused on telling a story or immersing the player in a plot, designed to feel as realistic (or fantastic) as possible.
Massively multiplayer online role-playing game (MMORPG) is a genre of computer role-playing games in which a very large number of players interact with one another within a virtual game world.
One of the first and most successful
- Second Life: http://secondlife.com/
- SecondLife -- Quick Start Guide: http://secondlife.com/support/quickstart/
SLurl provides direct teleport links to locations in Second Life. URL Structure:
- SLurl (basic API, URL parameter access to location links): http://slurl.com/about.php
- Example link: http://slurl.com/secondlife/Ahern/50/50/50/?title=SkipSearch%20ROCKS!&img=http://bcmoney-mobiletv.com/templates/images/logo.png&msg=BCmoney%20MobileTV
- WorldOfWarcraft: http://www.worldofwarcraft.com/
- WoW Simulator - What do you want? : http://elitistjerks.com/f32/t38686-wow_simulator_what_do_you_want/
Habbo (also known as Habbo Hotel) is a social networking website aimed at teenagers. The website is owned and operated by Sulake Corporation The service began in 2000 and has expanded to include 32 online communities (or "hotels"). As of June 2008 over 118 million avatars have been registered. There are an average 8 million unique visitors monthly, and 75,000 avatars are being created every day. The 100 millionth avatar was created on Habbo UK in June 2008.
- Habbo: http://www.habbo.com/
IMVU is touted by the service itself as "a new way to hang out and have fun with your friends online".
- IMVU: http://www.imvu.com/catalog/web_landing.php?p=az1&source=azoogle&c=38261-633z48z1z654she&medium=cpa&campaign=banner&content=darkgirl4_300x250
- Virtual World Learning Spaces: Developing a Second Life Operating Room Simulation: http://www.educause.edu/EDUCAUSE+Quarterly/EDUCAUSEQuarterlyMagazineVolum/VirtualWorldLearningSpacesDeve/163851
- VastPark promo (uses IMLL open virtual world language): http://www.opensocial.com/
- VastPark Dev: http://vastpark.org
- HTML5 Virtual World demo: http://kataspace.sirikata.com/ 
- wikipedia: Virtual world
- wikipedia: Second Life
- wikipedia: World of Warcraft
- SECOND LIFE -- 3D MODELS OF BLOOM’S TAXONOMY AND WENGER’S COMMUNITIES OF PRACTICE: http://pkab.wordpress.com/2008/11/26/model-3-dimensi-taksonomi-bloom/
- How do we know that we're not living in a computer simulation?: http://www.quora.com/How-do-we-know-that-were-not-living-in-a-computer-simulation
- Defining and Understanding Virtual Worlds -- By Karl M. Kapp: http://www.astd.org/LC/2007/0507_kapp.htm
- wikipedia: MMORPG
- wikipedia: Habbo
- Kataspace -- browser-based virtual worlds built with WebGL and HTML5: http://arstechnica.com/open-source/news/2010/12/kataspace-browser-based-virtual-world-built-with-webgl-and-html5.ars