The Arrow Tool (depicted above) is for selecting lines and fills (or parts of them), individual objects, or groups of objects. You can click or double-click on something to select it using this; select a fill and drag it free of its outline or vice-versa, or click and drag to draw a square outline to select portions of any line or fill.
The options for the Arrow Tool include Snap to Objects (which...well...snaps it to objects), Smooth (which will smooth the curves of a selected shape/line for one iteration), and Straighten (which will straighten the outlines of a selected shape/line for one iteration).
