Create a Texture
In the first Step, we'll create a Texture for the upcomming Terrain.
The Image Size should be 2048*2048 or higher.
Here's my Example Terrain Texture:

Create a Plane in Blender
To create a Plane, press "Spacebar" and select "Add->Mesh->Plane".(Picture#1)
After creating, scale the Terrain with the "s" Key.(Picture#2)
Now we must Subdivide the Terrain, because later we want to Deform this.(Picture#3)
Go into the "Edit Mode" press "A" to select the Plane. 
Now Press "W" and select "Subdivide", make this 8 times or less.

Apply Texture to the Terrain
In the first Step, we#ll go into the "Material Mode" and create a new Material.(Picture#1)
Now we create a new Texture. (Picture#2)
Apply the "UV" Flag to the Texture and set the Specular color to Black. (Picture#3)
Go into the "Texture Mode" and add a new Texture.(Picture#4)
Now it should look like this.(Picture#5)
Switch into the UV Mode and add a new Image.(Picture#6 / Picture#7)
Switch into the "Edit Mode" and press "7" on Num Block to set the Camera to the Top View. 
Now press "U" and select "Project from View (Bounds)" to create the UV Mapping.(Picture#8)
Finished... We have added a Texture to the Terrain.

Deform Terrain by Texture
Swith into the "Sculpt Mode". (Picture#1)
Now deform the Terrain with pressing the Left Mouse Button to increase the Height, 
press the Right Mouse Button to decrase the Heigth.
If you want to edit the Brush Size, press "F". To increase or decrease the Brush strengt, press "SHIFT+F".
Also you can change the Brush Mode (Default is Draw). Here the Brush Options:
-Draw (Key: D)
-Smooth (KEY: S)
-Pinch (KEY: P)
-Inflate (KEY: I)
-Grab (KEY: G)
-Layer (KEY: L)
-Flatten (KEY: T)

Load Models into the Scene
Importing Models into your Scene is very easy. Go to "File->Import->And Select your Fileformat".(Picture#1)

Model Material Properties
Now well set the Model a Texture with one Alpha Channel.
First, select the Model,Go into "Edit Mode" and create a new Material with the Properties.(Picture#2).
Then load the Texture File and load the same Image into the "UV Mode". Now it should looks like this.(Picture#3)
Load more Models into this Scene.
Now it could be look like this.(Picture(#4).

Finished
And here are the final rendered Results...