Hello and welcome to MinecraftSeeds.info!
The Minecraft 1.3 update added world generation seeds which are used to provide a starting point for the terrain generation algorithm when creating a new world. When used, seeds always generate the same terrain, therefore they can be shared with other players to obtain identical worlds.
MinecraftSeeds.info will be used to share a selection of the best Minecraft seeds which you can use when creating your own world. For more information on seeds and how to use them click the 'What are seeds?' page above.