Virtual Worlds, OpenSim, Grids & Hypergrid

What is a Virtual World?

A virtual world is a computer-simulated three dimensional environment populated by multiple users.  Each user is usually represented by an avatar, which is usually a graphic representation of a human, a creature or robot.  The multiple users can generally communicate with each other, usually in normal voice (wearing a headset with earphones and microphone).   
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Second Life is probably the best known virtual world.  It has been in operation since 2003 and has many of the features that people are accustomed to use in the "real" world.    That includes a virtual currency, a market for virtual goods, live entertainment, video production, P2P communication in voice and the ability to modify the environment so that people in such a world do not merely look at the enviroment, but also act on it.   

You can visit Second Life by going to the website and creating an account.  But if you do that, you will find that there is a lot to learn before you can do anything.  Here is a collection of web links with information about getting started in Second Life or similar virtual worlds.  

There is a video that will help you go into Second Life at least knowing how to move around and find out things.

OpenSim, OpenSimulator

These terms are used interchangeably.  OpenSimulator is a collection of open-source  software. that creates virtual worlds that are similar to the virtual worlds created by Second Life.  Open-source, however, means that the source code is available to the public and anyone is free to use it as is or modify it as they wish.  There are some restrictions on use, but basically anyone with enough technical expertise and/or patience can get their own copy and run it on their own machine.  Assuming they have a powerful machine, they can run multiple virtual worlds on their own machine.   

Grids

People have used the OpenSimulator code to create grids open to the public.  A grid, in this case, is an interconnected collection of virtual worlds.  Some grids, such as Kitely, are commercial and charge for land and maintenance services they provide.  Other grids are maintained by individuals or groups for special purposes such as education, role playing, etc.   Interconnected means that avatars and the users behind them can generally move from one virtual world to another.  A specific virtual world is commonly called a sim, region, or world. This movement is called a teleport (TP) and is technically similar to loading one browser page in place of another.   

Hypergrid 

The Hypergrid is a set of conventions that allow communication between grids running  OpenSim software.  The communication lets avatars and virtual objects move from one grid to another.  Many of the OpenSim grids are connected this way, so that people on the Hypergrid regularly visit multiple grids whenever they please.  From the user's standpoint, the collection of grids on the hypergrid is much like one large grid.  

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Selby Evans (Aka Thinkerer Melville in Second Life)
Selby Evans in Kitely
http://virtualoutworlding.blogspot.com/