Rectangular Marquee Tool

The Rectangular Marquee Tool creates rectangular selections:

1. In the Toolbox, select the Rectangular Marquee Tool
2. Drag it on your image holding down the left mouse button.
3. While the New selection option is chosen on the Options bar, you delete the existing selection by making a new selection.
4. To add an area to the existing selection you have either to choose the Add to selection option on the Options bar or to hold down [Shift] key while selecting.
5. To subtract an area from the existing selection you have either to choose the Subtract from selection mode on the Options bar or to hold down [Alt] key while selecting.
6. To intersect an area with the existing selection you have either to choose the Intersect with selection option on the Options bar or to hold [Shift+Alt] keys while selecting.
7. To create a selection with the predefined proportions choose the Fixed Aspect Ratio style on the Options bar and set Width and Height values.
8. While creating a new selection you can constrain proportions to 1:1 (ie. create a square selection) by holding down [Shift] key while selecting.
9. To create a selection with the predefined size choose the Fixed Size style on the Options bar and set Width and Height values.
10. While the New selection option is chosen and the cursor is within the selected area the tool looks like this and can move the selection (ie. borders of the selected area and not the image content). You can also move the selection using the keyboard keys.

Tip. Just in case you didn't know, the Rectangular Marquee tool is also available in Photoshop Elements that is way cheaper version.

And if you believe, Photoshop Elements lacks some other important feature, be sure to visit this page.