Straight lines alone are a useful thing, but it's not all. Actually they are not lines but curves. Cool anti-aliased smooth curves.
The curves can be modified quite flexibly by adjusting the 'handles' of the nearest points. 'What handles?' you say... See the image above? The small circles are those familiar points we used with the box in the previous section. The little boxes are those handles. The handles are not visible by default, you must drag them 'out' from a point. To make the handles visible 1. click to a point to make it active, 2. while pressing the Ctrl key, click again on the same point and 3. drag the handles out with the mousebutton still pressed down. You notice the curves between the points are not straight anymore. You can control the shapes by dragging the handles around