several tools that allow users to take screenshots of the world. I didn't take that pic, my friend did, so I'm not sure which tool he used, but there is a list of tools on minepedia here:
http://minecraftwiki.net/wiki/Programs_and_Editors#Mappers
The world is generated as users move around. I don't know how it is saved exactly, but I have a feeling that it gets saved once it has been generated, so the large expanse of untouched green areas in the shot I posted are probably a result of me going for a walk and getting lost.
Apparently Roller Coaster Tycoon was one of the games that inspired the developer. Reminds me of Sim City 2000 also.