Google it, look in a good saltwater fish book, check magazines. there are so many different kinds of skimmers, brands and sizes and where they areplaced.
Basically your skimmer creates a column of fine bubbles inside the cylinder that solid wastes attach to and areremoved from the water. You empty the collection cup every day or so and it is filled with bubbly, smelly, usually green yucky stuff that would still be in your water if not skimmed out. Do some research before purchasing one. Some are worthless and difficult to adjust, some are terrific-usually directly related to price.
I have a Euro-Reef and it is WONDERFUL. has been running nonstop for over two y ears now except for periodic cleanings.