Second Life is a virtual environment, better known as a MUVE.
"It is the largest 3-D virtual world created as imagined by its residents."
Second Life began in June 2003 and continues to grow. It has its own currency the Linden Dollar. Accounts are free, but you will eventually need money to acquire virtuals goods and to purchase land where you can build from a tropical resort, to a university to a house. You may also want to build houses and apartments that you can then rent for a profit.
Although 5-1/2 years-old now SL is still growing and developing. Many universities from Princeton to Yale and now the University of San Diego have a presence in Second Life.
There is also a Teen Grid of SL and it is strictly for 13-18 years-olds. At 18 your account can be transferred to the mature grid of 18 and over.
Create an account
It's free to get started. Only if you want to build and buy land do you need to add money to your account. If you are just exploring no need to spend money.
Places to Learn Second Life Skills