Optimising geometry

• Backfaces are bad, because they waste renderer cycles and risk creating glitches where a face shows over a face it should be behind. If you're building your model using geometric primitives, be sure to remove all the internal faces. In Blender 'local view' or use of layers help immensely.
• All faces are triangles as far as the engine is concerned. A more complex face is reduced to triangles.
• Vertices and edges don't matter as such. If you create three vertices, create edges between them, divide those edges into parts then create a triangle between the original three vertices you still have a triangle. In Blender, when you select a face the delineating vertices will be highlighted - if there are three of them, it's a triangle :-)
• Removing double vertices helps to get rid of extra faces

There is therefore a balance between culling backfaces and minimising face count.