The standard selection is actually part of the image, if it is edited, the original image will be changed. The floating selection hovers above the image, editing it will not change the original. If you have the layers pallet open when you are making a floating selection, you can see a button labeled Floating Selection. These options are ways that you can modify the selection to achieve a desired effect. This can not be used after the selection has been made.
All pixels selects only pixels no transparent areas.
Hold the Right mouse button down on the selection and move it to a new area. This deselect the previous area and reselects the new area in the same dimensions. This contains information about the color elements. Defloat it to paste it into the image layer again. The areas do not have to be adjacent to the selected colors.