Color Picker Tool

Sampled (or picked) colors from the Color Picker Tool are stored in the swatch next to the tool's icon on the Color or Swatches panel.

The following settings can be adjusted from the context toolbar:

Apply to Selection—when selected (default), the picked color will automatically apply to the selected object's Fill or Stroke, depending on which color selector is active on the Color or Swatches panel. When this option is off, the picked color is not automatically applied.
Source—determines from where the tool can pick colors—Current Layer restricts picking to the currently selected layer or object; Global picks from all objects/layers, irrespective of layer and selection.
Radius—Choose to sample from directly under the cursor (Point 1x1) or from differently sized square areas where the color is averaged.