Quote: "what is the different in dark gdk between a level and a terrain?"
Depends on what you call "level". For me, a level is a scenario: the collection of all objects, the whole environment that you use to represent the game environment. This can contain a terrain and can contain other object models.
A terrain, on the other hand, is a special type of object which has been invented to represent ground in your game, including hills, valleys... generally, the landscape. The terrain is usually composed of a lot of triangles and height information for each vertex, to build up the ground, and one or more textures to cover the ground.
The file type actually depends on what modelling program you use to create your environment. The dbo is the "native" file format of Dark GDK but it can also load other formats, like X and 3ds, which are useful if you want to import models created by modelling programs which do not know the dbo format.