So are Tacks. So are the outputs of gates. Valid destinations include circuit Outputs, inputs of gates, and Tacks. You can draw as many wires as you want from a source, but you can only draw one wire to a destination. When you are drawing wires, you'll notice some helpful visual clues.
Sources generally have a purple sort of color.
Destinations are green. When you are drawing a wire and you move the mouse over a legal destination, the component will be hilited. Note that some components, such as gates, have multiple inputs. When you move the mouse over a component with several available inputs, the wire will jump to the nearest one. Make sure that you get the wire attached to the one you want.