An open top rain barrel is, as the name implies, open at the top. This is so you can clean the inside of the barrel. It also allows you to direct water into the top of the barrel, which is good for many rain water harvesting set ups. A closed top rain barrel has advantages, too. With a closed barrel, if you position your diverter above the barrel, it will send water back into the gutter system automatically when the barrel is full. This will continue to work even if the barrel settles lower, which is very likely.