Not sure why there's a lack of serious answers, but there are a lot of approaches. If you're doing terrain you're probably using multiple textures and masks for most of the details, and choco's tutorials are good. You can make the actual terrains in many different ways though, they're much more free-form than most things. I've actually been considering doing all my terrain work dynamically, basically sculpt with dynamic topology in something like Sculptris and make a game mesh with with the reduce tools. In something like UDK you're using heightmaps to describe blocks of terrain in their landscape system, and the mesh is generated dynamically based on the level of detail the player needs to see. As I said choco's tutorials describe that system. The heightmap isn't the end of the story, you can make individual terrain mesh elements howerver you'd like, anything that over hangs or is too steep will probably be best off that way (cryengine has a voxel system but making actual mesh elements has more control).
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Just some ramblings, anything specific just ask.