The Battle Frontier is a place in Hoenn that is basically a Theme Park of Pokémon Battling

First, you need to sign up at one of the man desks in order for you to be able to freely roam the Battle Frontier. He will give you the Frontier Pass and you can go
There is a Special Shop in the Battle Frontier among the many people and huts. Click Here For Details
There is also a Cave called the Studio Cave which allows for quick access across The Frontier. Click Here For Details

In it there are 7 different Battle arenas. Click them for further details:

Battle Dome Basically a Tournement with 16 people
Battle Pike Shaped Like a Giant Seviper, you randomly pick an area and certain effects happen
Battle Factory A Place of Battle with Random Pokémon
Battle Arena A Place of Battle where you are tested on the way you battle...not if you win or lose
Battle Pyramid A Pyramid consisting of Mazes in order for you to get to the top
Battle Palace The Battle Palace. A Friend Battle Place, with a Twist
Battle Tower The Standard Battle Tower, with Four Modes, each varying in Strategy