The Triangle tool allows you to draw a right triangle (as opposed to the isosceles triangles produced by the polygon tool). Simply click and hold down the mouse, drag until you get the size triangle you want, then release the mouse button. The corner where the mouse was released will be the location of the right angle of the triangle.
The stroke width option allows you to specify how thick of a border you want, while the current Stroke Color will be used for the color of the triangle's border.
Elements drawn with the triangle tool can be filled with either a solid color or a gradient. You can also apply a drop shadow to triangle elements, as well as bevel/emboss them, or add a glow to it.