what is it? what does it do? where is it installed? how much does it cost?

Don't u mean nos....or to be correct Nitrous Oxide? N2O? if that's what you're asking in short nitrous oxide carries more oxygen in it's molecules than air, so your engine gets more oxygen to react with, so more bang, more power....
SEARCH lol

it also puts alot of pressure on your engine though

and if it is not installed properly, you may also go bang with the rest of your car!