"Second Life is a 3-D virtual social networking world entirely built and owned by its residents. Since opening to the public in 2003, it has grown explosively and today is inhabited by a total of 6,240,591 people from around the globe." 
The name of this virtual world results from the fact that each user can create his or her own resident or avatar. Once they choose an alternative version of themselves that pleases them ("Second life" offers a large degree of freedom in choosing one's appearance), users have absoulte control over the way in which they interact with other avatars in this parallel world.
As Donald Jones argues in his essay "I, Avatar," in Second Life you have the opportunity to be your perfect-self. It allows for people with disabilities to interact in ways that they cannot in real life, or just makes you look the way you always wanted to be. The two ways in which the avatars maifest this human need of having a mental image of an "ideal me" is by the fact that most avatars are either extremelly attractive or impossible fantastic creatures.
Second Life is unlike any other MMO because its goals are entirely user-determined. As a result, the world of Second Life has been built from the bottom up to cater to almost anything a Second Life user could want, from creating original art and attending lectures to greed and lust. The goal in Second Life, if any, is to participate in socialization, commerce, and creativity. Some people and companies use this virtual world as a commercial tool. Many large businesses (Reebok, Apple, and Nike, for example) have established headquarters in Second Life that advertise their real life products. People can even make money by selling their intellectual property--houses, vehicles, land. Creative people use animation programs in order to build elements and then sell them, both for virtual and real money. Owning property enables a sense of place and self, a very real feeling. Jones discusses this by describing it as the "rip, mix, and burn" reality, "Second Life" takes reality and bends it, alters it, so that it is recognizable as our own life while keeping it a fantasy at the same time. Most actions that avatar perform are very similar to our daily experiences (they eat, work, have sez, make friends, buy and sell stuff..)At the same time, all avatars can FLY wherever they go, and they can also teletransport.
Donal Jones: "I, Avatar: Constructions of Self and Place in Second Life, and the Techological Imagination"
http://www.secondlife.com/ Second Life Homepage
Donal Jones: "I, Avatar: Constructions of Self and Place in Second Life, and the Techological Imagination" http://gnovis.georgetown.edu/articles/FA05-01-secondlife.pdf