Seed (level generation)

Minecraft seeds are values made up of character(s) (including negative or positive integers) that are used as the basis for generating every Minecraft world.

World generation
Whenever the game has to generate a new world, it calls upon an algorithm. This algorithm will output a pseudo-random value that is then used to determine what the world will look like. However, the algorithm will always end up with the same value if the starting point (seed) that the algorithm uses is the same number. This is why seeds exist—to easily generate entirely different worlds from a single value.

Whenever the world generation algorithm is updated (usually by adding new biomes to the game), the same seed will no longer generate the same terrain. This is why in some different versions of the game, the same seed will not give the same world.

A world's seed is set when that world is created. By default, it is decided automatically (if so, the game uses the system time as the seed), but it can also be set manually. Set and reuse a seed to replay that world, or use a known seed to play the same world as another player. Either a number or a word/phrase can be used, including negatives. If a word/phrase is used it will be converted into a number.

If the seed or generator changes in a saved world, new chunks will be based on the new seed, and will not match those from the old seed (this phenomenon was widely observed with the Adventure Update). Deleted chunks can regenerate if the seed and generator remains the same, but will change if either the seed or generator changes.

It is important to note that since seeds are simply random values read into an algorithm and not actually names of different worlds, using a certain seed will not necessarily result in a world with any relevance to the value of that seed. For instance, using a biome name as your seed will not necessarily result in the creation of a world with primarily that biome.

Determining the seed
To view the world's seed, the player must enter the command. On Pocket Edition, the seed can be found on the world options screen.

Technical
The Java function is used to generate a number seed if a word/phrase is used. This restricts Minecraft to a subset of the possible worlds (to 232 which is 4,294,967,296), due to the datatypes used. Use number seeds directly or let Minecraft generate the seed (it uses the system clock) to access the full set of possible worlds (264 which is 18,446,744,073,709,551,616). Multiplayer seeds may only have 248 possible values if randomly generated by Minecraft due to using http://docs.oracle.com/javase/7/docs/api/java/util/Random.html#nextLong%28%29.

Pocket Edition
Pocket Edition has a total of 232 (or 4,294,967,296) possible worlds no matter whether strings or numbers are used as the seed. The same applies to the 64-bit version of Windows 10 Edition. This is because Pocket Edition uses a 32-bit variant of the Mersenne Twister PRNG, which will only accept 32-bit seeds.

Trivia

 * Certain seeds, such as 107038380838084, will return 0 in the internal code, causing infinite arrays of caves and other structures to generate.