Canvas provide a surface for drawing arbitrary graphics. Following example paints a circle on the canvas.

Java Code:
final Display display = new Display();
final Shell shell = new Shell(display);
shell.addPaintListener(new PaintListener() {
public void paintControl(PaintEvent event) {
Rectangle rect = shell.getClientArea();
event.gc.drawOval(0, 0, rect.width - 1, rect.height - 1);
}
});
shell.setBounds(10, 10, 200, 200);
shell.open();
while (!shell.isDisposed()) {
if (!display.readAndDispatch())
display.sleep();
}
display.dispose();