It has two main drawing tools: a pencil and a highlighter. Choose the pencil's thickness and color and start drawing. The highlighter only lets you change its color. Other tools included are an eraser and text.
You can export the result in different formats, among them PDF, HTML, and JPEG images. If you want, you could even use Jarnal over a local network as a collaborative blackboard.
Requires Java 2 Runtime Environment 1.4.2