The Bezier tool is like the Pen tool in Illustrator. You use it to draw Paths. To create a path, you have to make a click, then click again in another spot.
With every click you create a Node. A single click creates a sharp node. If you want to make it curve, then you have to click, hold down the left button of your mouse and drag.
The lines that appear are called Segments; the green one is a finished segment, the red is an unfinished one. If you want just to keep the green one, press the Enter key; if you want to include the red one, double-click your mouse.
To undo the previous segment, press Backspace.
Tags: bezier tool, Inkscape tutorial