Color Palette
...a mini version of the color picker, it is less precise but may be quicker to use. It allows you to select foreground and background colors by clicking on the foreground/background icon in the palette. As with the color picker, you can specify colors numerically as well.
Swatches Palette
... contains 122 swatches from the image. It also allows you so save a color to the palette.
Color Picker
... allows you to select the full range of colors the monitor is capable of displaying, and also for selecting color numerically. It's invoked by double clicking on the Foreground/Background color icon on the bottom of the Toolbox.
Eye Dropper Tool
... allows you to select colors from the image itself and can be temporarily invoked by pressing the spacebar while using any other tool. The new color can also be "carried" to the swatches palette by the paint bucket tool and saved.