Knowledge (geography) deals with not only the locations of things, but also knowledge of the general terrain, climate, and peoples of a given area. Specializing in this skill means the PC has intimate knowledge of a given area – a nation or region up to 1,000 square miles in size. He can name and pinpoint even small towns, landmarks, or other points of interest that don't appear on most maps, can answer questions about most sentient humanoid races in the area, and knows the terrain and the predominant climate for a given time of the year.
This skill also depends on how well-traveled the world is; on some worlds, the PCs know of and travel on only one or two continents, while on others, they go all over the globe. A rule of thumb here is that knowledge of another continent/region increases the DC by 5 if it is close (North America and Central America, for instance), 10 if is further away (North and South America), or 15 or more if it's on the other side of the world (North America and southeast Asia). The DC is also increased by 5 if the world is well-traveled (all continents have been explored), 10 if it is moderately traveled (most continents), 15 for somewhat traveled (the continent the PCs are on and one nearby), or 20+ for barely traveled (the one the PCs are on, or even just a small portion of that).
These numbers are only guidelines – they can be adjusted according to the campaign world. For example, if a mage order has set up a wide network of portals to enable fast transport to most places on the globe, or people can teleport along ley lines, then the DCs would be significantly lower, as information would flow more freely. On the other hand, if most of the world is covered in heavy forest, or there is little magic and people don't travel much, the DCs would be increased by 10 or more.
"Region", as stated below, is a fluid term – there are no real guidelines for defining a region, but it should be no more than 500 square miles.
This skill is contrasted with Knowledge (local), which is applied to a specific area up to 50 square miles in size.
On a DC 5 check, the PC can place a metropolis (as defined in the DMG, Chapter 5) on a map, to within 10 miles of its actual location. The DC goes up by +3 for each smaller size population center (DC 8 for large city, DC 11 for small city, etc., all the way to DC 29 for thorp).
The above also applies to landmarks – large or well-known landmarks like the tomb of a famous hero or past ruler, or a major geographical feature like a mountain, would be DC 5, increasing in difficulty to small or obscure things like a silver mine that played out a hundred years ago and has been closed ever since, or a time-worn obelisk deep in the forest.
On a DC 10 check, the PC can name the major sentient races in the region. For each 5 points over DC 10, he can recall one extra piece of information about a given race.
On a DC 10 check, the PC knows the general climate for a given time of year in a region. On a DC 15 check, he can tell whether or not the area is prone to unusual activity – tornadoes, hurricanes, earthquakes, etc.
On a DC 10 check, the PC knows the general terrain of a given region – plains, mountains, hills, forest, etc. On a DC 15 check, he can give specific information – heavy forest, high mountains, rugged hills, etc.
On a DC 10+ check, the PC can name major nations, cities, or landmarks, and knows where in the world they are located. The DC is 10 for large nations, or those closest to his home, and increases the further away and/or the smaller they are.