If you use a computer very long, sooner or later you will accidentally put an item in the trash that you didn’t mean to throw away. Assuming you have not emptied the trash, you can get back.
It used to be the case that the only way you could retrieve it would be to click on the trash can to open it, search for the item, and then drag it back onto your desktop or back into a specific folder.
With Snow Leopard there is a somewhat faster way.
Right-click on the item and select “Put Back” from the contextual menu. The file will return to the where it was before you threw it away.