I call them Device Canvas or Popup. Device canvas is the main area where a device menu will appear. A popup is where a series of buttons would appear. The popup can be fixed or made visible by pressing a button. See the examples below.
With my GUI the device canvas is always the same size and located in the same area. This keeps the page layout consistent and is nice when you like to have everything on subpages in a Crestron project. Its the rectangle (in this example) in the background. The idea is to keep a devices menu inside this area.
This is an example of using a fixed button to bring up a popup. It is translucent in this example.
This is an example of a fixed popup. Its technically not popping up but i decided to call it that vs dialogue box or rectangle. The terminology isnt really important i guess. I had to come up with names for things when working on projects with other people because you get tired of hearing "can you make the box bigger" WHAT BOX? LOL.
Ok finally to your question.. The easiest thing to do is to simple drop a shadow or glow under a shape. If you want to get fancy you can add some translucency to the shape. If you want to get even fancier you can add more effects to the shape.
Box 1 and 2 are from my Tactical template. Box 1 is solid Box 2 is translucent. They are identical except for the translucency. If you look at box 1 you can see i added some shading here and there and a glow under it to make it appear like its floating.
Box 3 and 4 are white and black plain shapes with the transparency set to 50%. They both have a glow effect added to them. These are probably the easiest to make.
Box 5 has some simple effects but it looks really good. It has a slight gradient with the bottom being darker. A light blue trim and a glow effect. I also added in some feathered ovals at the top and bottom to give it a little more depth. YOu can see all the shapes involved in making it below it.
Look around in your graphics editor for something that will drop a shadow or a glow under an object. Thats probably the easiest first step.