Is 1 object = 1 mesh exported? For example I create 1 cube and extrude as much and as many things as I want out of it, but then I need to add a new cube to add a simple thing which couldn't be easily extruded from current mesh, so I do it... 

Does that new cube get exported as separate mesh or are they joined into one mesh when exporting? And if it's not the latter, what would be the simplest way (if possible) to join them and delete faces which would be obstructed?

I've never used blender before (nor any other modelling software) so I have no idea how most stuff works in here.
The answer depends upon how the item is designed. One object can be one mesh, but one object can also contain two or more meshes. In Blender, the answer depends upon exactly how the new cube is added. If it is added in object mode, the new cube is a separate object. If the new cube, though, is added in edit mode, it may be part of the original object, even though it is a separate mesh.

The answer also might depend upon what you are importing the object into after export. Without knowing what you are going to be importing the exported object into, one cannot say for certain. It might be that the two meshes will be imported as separate objects, or as a single one.