When saving a world it saves it on the the map item if placed in an item frame, and you keep all of the inventory you were using however when entering the world again the save has reverted and anything you have built disappears, for example I built a rail line from my house at the top of a mountain and sent it down 550 blocks, I think south in total and when I left the game and came back about half an hour later the line appeared on my map but not physically in front of me from where I left off yet I had all the stuff I was using from the previous save.