The Channel dialog lets you manage and pick up new colors. It is divided into five seperate parts: GIMP, CMYK, Triangle, Watercolor and Scales. You can use the eyedropper, which is the last button of the dialog, to pick up a color anywhere on you screen.
The dialog can be called in many ways :
The CMYK view gives you the possibility to manage colors from the CMYK color model.
The color picker has a completly different behaviour, than the color picker tool. Instead of picking the colors from the active image, you're able to pick colors from the entire screen.