Transparent windows, create transparent Form

You can do it by defining the BackColor of a color and that same color apply it as TransparencyKey :

 this.TransparencyKey = Color.Crimson;
 this.BackColor = Color.Crimson;

With this you would achieve what you want, the transparent form but not the controls:

The other option is to use the opacity property, Opacity but this applies to the entire form including the controls it contains:

 frmTransparentForm.Opacity = 0.5;

where 0.0 is full transparency and 1.0 full opacity.

This will make your Form transparent.

Here an article: How to create transparent forms.