Islands are geographical features distinguished by isolation from main continents. Island fortresses are characterized by inaccessibility to most traders and invaders. Only civilizations located on the island, and your parent dwarf civilizations, will be able to make it to an island fortress. The only exception to this is if the island is in close enough proximity to land, and in a Cold or Freezing biome, so that the ocean is able to freeze and caravans are able to cross.
Advantages of islands
- The lack of goblin and kobold civilization access makes your fortress a peaceful location to learn how to manage the game's less military aspects.
- No military interruptions makes it easy to build mega-projects.
- Easy access to a large body of water for underwater mega-projects
Disadvantages of islands
- The lack of invasions can get dull. An idyllic island paradise is all very well and good, but without constant slaughter your dwarves will have nothing to engrave about except the vermin they like least.
- No non-dwarven caravans on most smaller islands. You won't be able to collect bins of crappy cloth nearly as effectively, and won't be able to collect large bronze helmets at all.
- Megabeasts, titans, and forgotten beasts can still arrive on an island. This can be exceedingly fun if a lack of other invaders led you to get complacent.