Seeds are things you type in to generate maps but if you build on a seed and someone types that seed in, they will not see what you have built. When you create a world on single player go to where you'd set structures on or off . . . type in something above that. Here are some seeds!

hillsygirl101 will generate a beautiful forest with sugar cane and a lake like thing with hills
105.2 will generate a very hard to survive on map with only 3 trees