Second Life is the biggest online virtual world. It was developed by Linden Lab and was launched on June 23, 2003. Some people use Second Life for entertainment purposes, but it is not a game because it does not have any single goal that players must achieve. Anyone is free to do anything they want, as long as they respect the Terms of Service. They are also free to create new things and add them to the virtual world. Players can make places, clothes, and a lot of different objects. This is why Second Life is considered a platform, too.
With regard to gaming, Second Life does not have the same rules as other online games. For example, users can fight each other in Second Life, but either person can stop fighting any time and they do not really lose anything by doing so.
In Second Life, people can also do things that are about real life. For instance, many universities and companies are using Second Life for education and training.
In short, Second Life is what you want it to be.
Q: Why does SL run like shit?
A: Terrible viewer optimization, sculpts, and terrible object optimization. Lower your draw distance and object detail for free frames. Also try out avatar impostors.
Q: Which viewer should I use?
A: Firestorm is the go-to if you're going to model, script or build. Singularity or Alchemy for performance boosters and just to play, but it's up to you to decide what you should use.http://wiki.secondlife.com/wiki/Third_Party_Viewer_Directory
Q: How do I make money?
A: Provide people with a service or a product and someone will be willing to pay you. The only issue I see people running into is making themselves known.
Q: How do I make things for Second Life?
A: Any image editing software can make textures for SL so long as it can save in jpg, png, or tga format. Image resolution must be in powers of two (maximum being 1024, some viewers do accept 2048 but SL will downscale it). Height and width don't have to match. There's an in-game primitive manipulator and editor. It's really limited but you can make basic structures with it.
There's sculpts too, generally speaking they're terrible but they have their advantages. (http://wiki.secondlife.com/wiki/Sculpted_Prim)
Only consider using them when you're using mesh with a ton of prim states. Mesh objects can be made in nearly any 3D modeling program. Upload in .DAE format.
And finally there's Second Life's scripting language, LSL. LSL isn't as flexible as you think it is, though it is very powerful to do simple scripts in a complex world.http://wiki.secondlife.com/wiki/LSL_Portal