Answer 1

A Master Ball is something that can catch any sort of pokemon.
Even the rare and legendary.
Nothing could stop a pokemon from coming out of it.

Comments

Any WILD Pokemon.  Don't go throwing it at another trainer, or they'll block it and you'll just waste it.

Normaly there's one in each game, but I know that in Diamond and Pearl (I don't know if it applies to HG+SS) the lottery 1st prize in the TV building can get you another one, but the ID number  has to match exactly to one of your Pokemon's.

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yes,it is also the rarest pokeball in the game,so when you use it the only way you can get another is to get a action replay or somthing depending on the game

Answer 2

Catches wild Pokemon. NEVER fails.

Answer 3

The master ball is the perfect purple pokeball, that never fails to catch a pokemon.

Answer 4

The Master Ball is the poké-ball that never fail to catch a pokémon. On SS/HH, Prof. Elm give one to you.
But, if you want more, the only way I think it's possible to get them is by using Action Replay or trade pokémon in another game (like Pokémon Platium, for eg.) holding a Master-Ball.