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. The gates in the pallette can be rotated into four different orientations.