Rustboro City is a city in Hoenn. Roxanne's Gym is here and it is also the city that contains the Devon Corporation, which makes items for people inside the game (for example) the PokéNavigator, or the Pokénav for short. In addition, the Devon Corporation is also the area for Hoenn where fossils are reverted back into living Pokémon. Also, the house beside the PC (Pokemon Center) is where you can get the HM Cut. You need to defeat the gym leader Roxanne before you can use the HM Cut.
It is connected to Petalburg through the Petalburg Forest followed by a sea-side route from the south, connected to Fallarbor Town through Meteor Falls followed by a one-way trail from the north, and connected almost directly to Verdanturf Town through the Rusturf Tunnel from the east. It appears to be surrounded by a cave or forest on all three sides.