A Sheet panel is a collection of Sheet properties, that is shown when the Show / Hide Properties button is clicked above the project tree (icon of a list and a black triangle) or keys [ctrl]+[s] are pressed.
A Sheet window and a Sheet panel overlap, so that you may need to bring the one or the other to the front. Apart from using the Show / Hide properties' button, you can click a part of the window you need that is still visible. For the Sheet panel, that includes the project tree.
Some properties of the Sheet window can be set on the Sheet panel, such as its color, width, height and grid.