Every time you create a new world in minecraft, it will be assigned a random unique value, known as a seed. This seed is kind of like a barcode for minecraft saves, and allows minecraft players to share the cool worlds that they have found with other people. However, any changes made to the world made by the player will not show in a newly created seed.

You will not always spawn in the same location for every seed, sometimes you will spawn in random places of the same world.  When a new major patch comes out for minecraft, each seed is usually changed in some way, and so a seed made in version 1.1 of minecraft, may not be the same as in version 1.2.

Minecraft Seed Creation

To create a custom seed in minecraft, enter the singleplayer world selection at the main menu of the game, then press the “Create New World” option.

On this screen, press the “More World Options…” button and there you will be able to enter your custom minecraft seed for the world.