A person who absolutely loves fire. The smell of gas or smoke sends them into ecstasy. Lighting a candle and blowing it out and relighting it and blowing it out repeatedly, watching the smoke lightly twirl around in the air, is a good sign of an pyro. Many pyros can figure out different types of gas and/or what's being burnt just from the smell. Watching a flame on a lighter flickering is fun and addicting, but watching a full piece of paper burn all the way to the bottom is complete elation. Pyromaniacs are difficult people to understand and are often confused with arsonists. Pyromaniacs know how fire works, can predict in which direction it will burn, how much smoke the fuel will make, how quickly an object will burn, what the burning object will smell like even before it is being burnt, knows how to mix chemicals to make cool flames (like boric acid with alcohol for a green flame) and other crazy cool things. An 11 year old kid who wants to be bad ass in front of his friends and owns a lighter and makes what he calls "fireworks" (i.e. match rockets, penny bombs, etc.) is NOT a pyromaniac, just a fag.
11 year old: "Hey look guys! I just made a FLAMETHROWER out of an Axe deodorant can and a lighter!! haha wow I'm such a pyromaniac!!!"
Friends: "No, dude, you're not, you're a fag. Anybody can make a 'flamethrower' out of deodorant and a lighter. Can you make a pipe bomb? No? You haven't? Can you tell the difference between the smell of butane and natural gas? You can't?! Well then you're not a fucking pyro! Go home and play with your matches."