A Multi-User Virtual Environment is one in which many users are ‘embodied’ as avatars in a shared 3D space. Users communicate via text and voice chat and in some MUVEs have the opportunity to create 3D objects of their choosing.
See http://en.wikipedia.org/wiki/MUVE for an overview of some of the high profile MUVEs

Some of the biggest MUVEs in terms of membership are actually designed forchildren: http://www.kzero.co.uk/blog/wp-content/uploads/2008/09/universe-v6-maste...

This type of environment gives some users a positive sense of being co-presence with other people which goes beyond what text based systems can offer. For others MUVEs are strange, alienating and ‘full of weirdoes’. In fact, it's the effect of co-presence that makes individuals reactions to MUVEs so strong.

The Open Habitat project made extensive use of the Second Life MUVE which has some of the most advanced building and scripting functionality. The project also used OpenSim, an open source version of Second Life which allowed us to provide one ‘island’ each for the art and design students to practice building skills on.

If you want a pragmatic guide on how to get started using Second Life please download the Open Habitat & Preview project guide to Second Life. This should give you a good lead into the basic steps needed to get set-up and some straight forward tips on how to run teaching sessions.

To find out what the Open Habitat project believes MUVE bring to the elearning party see the post called ‘Why Not Use Something Simpler?’