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