Last night I taught a Composition in the Field class for the Digital Photo Academy. Unfortunately there wasn’t enough light to give my normal demonstration in the use of a lens hood. So I went out this morning, found a willing subject, and shot a with-and-without demonstration. You can click on the photos below to enlarge them.
Here is the photo shot without a lens shade with the sun in the upper left hitting the front glass elements of the lens as shown in the diagram below:
Here is the same shot with the lens shade. You can see that the tree trunk on the left has better contrast do to the fact that that orange streak of sun is not bouncing around inside the glass elements.
The lens shade blocks some of the light. 
If you don’t have a lens shade you can always use your hand or a piece of cardboard.