Preamble: This is a (slightly edited and updated) repost of an article I posted to one of the restricted forums. I'm reposting it here so that everyone can read it. When I originally wrote this article, I discovered that some of the functionality needed to make it work wasn't implemented in the game; these problems have been fixed in version 1.0.2, so most of you reading this article won't be able to try it out until you can get your hands on an updated version of the game. Anyway, here we go!
If you've had a chance to play GalCiv2, you've probably noticed that while planets in the game may look different, the game uses the same terminology to describe the different quadrants of every planet's surface. For example, every usable quadrant of every planet in the galaxy is called a prairie. Well, Mars doesn't have prairies. If this bugs you to the point that you'd change it if you could, read on!
In CustomPlanets.xml, you'll see that Mars uses the RedPlanet terrain palette. In PlanetDescriptions.xml, you'll find the RedPlanet planet description which (despite the difference in nomenclature) is what the TerrainPalette tag in CustomPlanets.xml is referring to. In turn, the RedPlanet planet description refers to the RedPlanet1 color scheme, which is defined in TerrainColorSchemes.xml. If you add a TerrainName tag to the the Override node of RedPlanet1's color scheme, you're telling the game the name of the string resource to use when naming that type of terrain in the game. So, if you add a TerrainName tag with a value of "MarsTerrainTypePrairie", the game will look for a string tag of the same name in the PlanetWnd section of strings.str. If it finds it, it will use the associated string instead of the usual "Prairie". To recap:
1. Add "<TerrainName>MarsTerrainTypePrairie</TerrainName>" to the Override node of the RedPlanet1 color scheme in TerrainColorSchemes.xml.
2. Add "[MarsTerrainTypePrairie] Usable Red Mud" to the PlanetWnd section of screens.str.
Voila! Now, next time you run the game (assuming you've gotten the next version with the fix that enables this mod), when you click on a usable tile on Mars (or any other planet that uses the RedPlanet1 color scheme), it will be identified as "Usable Red Mud" instead of "Prairie".
The basic process is the same for renaming other terrain types, though you'll have to create additional Override tags for each terrain type you're renaming. The comments at the top of TerrainColorSchemes.xml gives additional information about doing this, but you can simply copy the existing Override node, strip out the ColorDef tags and then edit the TerrainType, TerrainName and QueryPicture tags accordingly.
While this article doesn't spell out every last detail, hopefully it's enough to get you started. 
And, if you get stuck, you can always post a call for help in the modding forum.
Mod on!