Black City and White Forest
Each version of the game as an exclusive location between Routes 15 and 14 in Unova. In Pokemon Black there is the Black City, while in white there is the White Forest.
Black City has extra items that can be purchased. The available items can change by using Entralink and entering another player's City or Forest. The city also includes high level trainers that use Pokemon ranging from level 30 all the way to level 65. There are Backpackers and Roughnecks that have multiple Pokemon and will offer Triple and Rotation Battles. Black City is a good place for high end fights and finding difficult challenges.
White Forest contains exclusive Pokemon from older generations. Typically they are around level 5 when caught. As with Black City, White Forest can be changed by using Entralink to explore another person's City or Forest. There are also a handful of trainers here, but not nearly the amount in Black City. White Forest is better for catching older Pokemon that you may want to train up.