How to Use the Photoshop Marquee Tool
From Eric Miller, former About.com Guide
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Modify the Selection
Add to the Selection
After you have created a selection, you can modify it by adding or subtracting from it. Start by creating a selection on the canvas. To add to the selection, hold down the shift key and create a second selection. This new marquee will add to the first… as long as you continue to hold the shift key before each selection, you will add to it. To subtract from a selection, follow the same process but hold down the alt/option key. You can use these two methods to create countless figures, which can then be used to apply filters to a custom area or create shapes.