Press, drag, and release the left mouse button to create lines.
Press, drag, and release the right mouse button (Cmd-mouse on Mac OS X) to drag corners and line ends around.
Click Ctrl-LMB (Control/left mouse button) near a line to select it. Click the "Delete" button to delete the selected lines. Click the "Deselect All" button to deselect all selected items.
The "Redraw" button redraws whatever is currently on the screen.
The "Clear" button wipes the slate clean, removing the current drawing.