What is a skimmer used for?

[White Lightning]

Just out of curiosity, what exactly is a skimmer? i have seen a million ads for them but never bothered to ask what they are used for.

[BigDaddy]

It is used in saltwater setups to remove dissolved organic compounds.  Since you posted in the Freshwater forum, I will also say you can get SURFACE skimmers for freshwater that will remove a protein layer from your water if you have one.  Freshwater surface skimmers also increase oxygen levels because of their design.

[dpatte]

note: saltwater skimmers work by making millions of tiny bubbles, but that process doesnt work in fresh water.