A Master Ball is a type of Poké Ball that is able to catch any type of Pokémon without fail. It is a very rare ball and cannot be bought, it has to be given to the player.
The Master Ball was developed at Silph Co., though the project had to be discontinued when Team Rocket invaded their labs. Because of this, only a few Master Balls can be found within the world of Pokémon, and the known Master Balls are usually only given to the most experienced Pokémon Trainers. In the second generation of games, you could sell it to Poké Mart for nothing, though in newer generations they refuse to buy it. In addition to the one given, the Lottery gives a chance to get one though incredibly slim.
In the anime, the character Sullivan used a Master Ball on the pokémon Whiscash, though the creature simply swallowed it and escaped in the water without getting caught (despite the fact that a Master Ball, in the video games, will capture the Pokémon without fail). In another episode there was a beach ball that resembled a Master Ball.