What is the Pen tool good for and what is its keyboard shortcut?

The Pen tool is used for drawing paths composed from straight lines and curves onto the Artboard. The keyboard shortcut for using the Pen tool is P.

In order to draw a path, select the pen tool and start clicking on the canvas. This will add points that will be connected by straight lines.

You can modify the path by selecting, moving, adding, deleting and converting points.

• Select a point - To select a point in a path, double click the path and click the point you want to select. You can also select multiple points - just hold Shift key and click the points you want to select. 
• Move a point - To move a point in a path, double click the path and drag the point to a different location. You can also move multiple points - just click them while holding the Shift key and then drag them to a different location.
• Add a point - To add a point to a path, double click the path and click the path at some location. The point will be added to the location you clicked.
• Delete a point - To delete a point from a path, double click the path so you enter the editing mode, then select the point by clicking it. (You can also select multiple points by holding the Shift key and clicking them). Once the point is selected, press the Del key.
• Convert a point - There are two types of points. The point that connects two straight lines and the point that connects two curves. To convert between these two types of points, double-click the path and then double-click the point.

Each free-form path shape can have different

• Opacity,
• Fill, 
• Border,
• Shadow and
• Background/Object Blur.