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 into living Pokémon. Also, the house beside the PC (Pokémon Center) is where you can get the HM Cut. You need to defeat the gym leader Roxanne before you can use the HM Cut. Go inside the Poke Mart, and talk to the guy in the suit and tie. He will give you the TM False Swipe, which will be useful once you get farther in the game and your Pokemon will be too strong to weaken a wild Pokemon without making it faint. Also, go into the Trainer's School, then talk to the teacher to get a Quick Claw. Then, go in the house northeast of the school. The kid wearing a blue hat will trade a Makuhita for your Slakoth, which will be usefull for the gym.
It is connected to Petalburg through the Petalburg Woods 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.