The Brush tool allows you to draw on the screen and produce brush-like stroke. The current Stroke Color will be the color of the drawn path.
The Brush Size option allows you to specify how wide of a stroke is drawn. The Softness option determines how sharp-edged the stroke will be; higher values will appear softer. The opacity option lets you make the brush stroke appear transparent.
The Smoothing option allows you to control how much automatic smoothing is applied to any paths drawn. Maximum smoothing will remove many points in an attempt to make a very smooth curve (at the expense of accuracy). Minimum smoothing will leave all points in the path, preserving accuracy (what you draw is what you get).
The widget for brush elements also allows you to modify the shape of the stroke after you have completed it.