Backface culling is active by default.  Think of it as a sheet of paper; the side you are not using appears culled.  You can use ‘flip normals’ to flip which side you want to be the facing side.

You can turn backface culling off:

With backface culling off, the ‘sheet of paper’ magically appears to be double sided.

Backface culling is a “DISPLAY PROPERTY”.  It does not create additional polygons, it does not change how a texture is applied and it does not affect rendering!  In both situations the render looks like this:

Materials are applied 2-sided by default.  You can change this behavior as follows:

In the material editor

In the render setup:

Rule of thumb: stick with the defaults.  If you see weird polygons that you can’t select, just realize this is due to backface culling.  Think of them as the ‘non-facing’ side.  They help to visually inspect whether or not your normals are facing the right way.  Leave your materials 2 sided; it doesn’t matter if the inside of a hollow object is textured or not generally.  If you want the interior of a car visible, you have to model it first; everything has thickness – slapping a texture on both sides of a simple car mesh doesn’t give you a car interior/exterior just like that.

This is how a simple opacity map works:

Rather than creating 2 different bitmaps, you could use an image file format which supports transparency information for each pixel in the image (TGA / PNG).

PNG texture:

Material editor.  Notice the opacity slot is not used:

Render:

Material editor – Notice how the texture bitmap image (PNG) is used both in the diffuse slot and the opacity slot:

Note that you have to change the “Mono(chromatic) Channel Output” to Alpha, so the PNG transparency (and not the PNG RGB) channel is used -only for those material maps that typically expect a black and white (monochromatic) bitmap; the diffuse map will use the RGB channel of the PNG image obviously.

Render: