As you build your stronghold, you change and develop the land. It might start out as absolute wilderness full of monsters and baddies, and (with years of development) get transformed into a buzzing metropolis that dominates the surrounding nations. However, it goes through a number of different stages on the way. Each of which has their own special advantages and disadvantages.
Wilderness is how most land starts, home to wild beasts and monsters. You may find a few hardy travellers and trappers here, but that is about it. While you can claim real wilderness as part of your stronghold, nobody really recognises that you own the land and it doesn't count until you do something with it. Claiming it, however, does lay down a marker of your intentions.
Managed Wilderness is an area of wilderness that is patrolled by your troops to keep down monsters and barbarian tribes, however, nature is pretty much allowed to run its own course. The hex may have a single watch tower (or similar) that provides a base for the troops that patrol the area, although in some cases it could be patrolled from an adjoining hex. If your stronghold has a border with a real wilderness area, you can use Managed Wilderness to build a border area that is safer for those travelers and trappers. You may not build Roads, Highways or Canals in Managed Wilderness areas, although Trails will develop naturally. If a hex is held by barbarians, bandits or chaotic tribes it may already be a Managed Wilderness and have something you can take over as a patrol Base.
Semi-Wilderness is land with a few nature-friendly developments. There are enough people that the natural world is affected - but not so many that it makes huge changes. A Semi-Wilderness hex can have a road running through it, although you cannot build highways or canals in these hexes. A Semi-Wilderness corridor would allow you to build a road across a wilderness area to connect two settlements together. Rangers, Barbarians, Druids and other nature priests could use a combination of Semi-Wilderness and Managed Wilderness hexes to build a very wild, natural stronghold.
Rural land is covered in small holdings, farms, fisheries and villages and provides the the everyday food and resources that your stronghold needs. You could build a nice pastoral countryside domain with a mixture of Rural and Wilderness hexes - brilliant for anyone who doesn't want to live in the city.
Urban developments means you are building a Town, that might develop into a city or even a metropolis. While the city itself doesn't take up the whole hex, everything in the hex is affected when you start to build a town. The land surrounding the city counts as suburban - and has all sorts of supporting industries spring up (Villages, Farms, Fisheries, quarries etc.) spring up without you doing any thing. The bad news is that you might lose a lot of the benefits from some earlier developments (if you aren't careful). A traditional Barony has a town in the middle surrounded by a combination of rural and wilderness hexes. However, that isn't always the case and an Independent City might just take up the single hex it occupies.