The Attribute Dropper tool lets you copy the fill, stroke, and live effect attributes from one object in your document to another. Essentially, in one movement, you can copy all the formatting you can set in the Properties panel.
To copy attributes using the Attribute Dropper
In the Toolbox, select the Attribute Dropper.
Select the fill or stroke area of the object that you want to copy.
Hold down the mouse button and drag the Attribute Dropper tool to the second object.
Release the mouse button to copy fill, stroke, and effect attributes to the second object.
If the second object has no fill or stroke color, then release the mouse button when the tool is over the path itself.