1. Open Photoshop. Open Image to work with rectangular marquee tool.
2. Select Rectangular Marquee Tool from toolbox. Drag on the image. You will get animated selection border. It indicated the area selected by the tool.
3. Press Shift and drag another rectangle overlapping some part of the previous one. Now release shift key. You will get the addition of the two selections.
4. If you press Alt key and drag second selection, the overlapping part will get subtracted from the first one.
5. You can try similar technique using Elliptical Marquee Tool also. In the following image we have used Elliptical Marquee Tool and shift key to add two selections.
6. In the following image we have used Elliptical Marquee Tool and Alt key to subtract one from other selection.