Rectangle and ellipse select tools

The role of the Rectangle Selection tool is to select rectangular regions of the active layer.

The role of the Ellipse Selection tool is to select both circular and elliptical regions from an image/ active layer.

The Shift key 

Pressing the Shift key after starting your selection, activates the Fixed option. If you keep holding the Shift until you finished the selection, the resulting selection shape will be a square or a circle ( depending on the selection tool used).

When you have already done a selection and you start a new one, if you press the Shift key before starting the new selection, the resulting selection will be added to the existing one. 

A "+" appears on the cursor. 

The Ctrl key 

If you press the Ctrl key after starting your selection and you hold it down until you finished, you will change the centre of your selection. More specifically, instead of using the corner as your center, it will use your starting point.

When you have already done a selection and you start a new one, if you press the Ctrl key before starting the new selection, the resulting selection will be subtracted from the existing selection. 

A "-" appears on the cursor. 

The Shift + Ctrl keys

Pressing these keys before starting your selection intersects the resulting selection with the existing one.