Man, that's hard to answer quickly...
Put VERY simply, it's just combining a million different photos and images in photoshop haha. (ok - hundreds, not millions). Some bits I draw directly in photoshop myself, eg seaspray, smoke and that ridge right in the middle of the desert are just drawn with paintbrushes.
There must be about 10 different photos of skies for example (that's before you get into lightning territory). It's basically finding a way to combine lots of different stuff together to make it look cohesive.
Each indiviual image is altered so much, from brightness/contrast, to sharpness/bluriness, to saturation of colour etc to make sure it blends in properly.
Then often, each image will be copied a few times and set to different layer styles (eg "hard light", "overlay", "multiply" or whatever) and then with different opacities for the desire effect.