The military base / town of Junon plays a pivotal role in the game and during a standard playthrough most players will end up visiting it a number of times. Junon is located on the western coast of the eastern continent and it is the last town that you visit before crossing the ocean over to the western continent in Disc 1.
The first visit to Junon is after the team emerges from the Mythril Mine and after the visit to the Chocobo Farm. The team leaves Junon by way of the Cargo Ship which then takes them to Costa del Sol.
The third and final visit, as far as the main storyline goes at least, is when the team passes through Junon on the way to the Underwater Reactor. This is when they are in pursuit of the third piece of Huge Materia and just before the Submarine Fight in the Shinra Sub.
There are a number of items that you can pick up in the various houses and apartments in Junon including:
There are also a ton of shops in the area. The shops have additional items once you have concluded your second visit at Junon once Meteor has been summoned by Sephiroth:
* Note that there are multiple Weapon Shops and Materia Shops scattered throughout Junon.
The map below shows the location for Junon on the World Map: