they are an explosive made by mixing some fuel (usualy liquid or gas) with an amount of air(specific ratios vary by fuel and conditions) then igniting it. a very simple one can be made by taking a 55 galon hefty bag and waving it around until it is full of air, then opening just the acetylene valve of your torch into it for a few seconds. tie it off and get FAR away put ear plugs in and fire a flaming arrow through it. quite an impressive bang huh?
modern fae weapons usually contain some liquid fuel in a highly pressurised aerosol container. the aerosol is released just before the bomb achieves the target then a when the fuel air ratio is just right the fuse sets it off. our government claims now that we only use them in mine clearing and antistructure purposes. that wasn't always the case. they are one of the most effective and horrible antipersonnel weapons imaginable. you can see a spiffy demonstration here
there have been allegations of the Russians using fae's in Groszny as antipersonnel weapons. They are likely true. These weapons are a powerful psychological weapon. the first thing the the weapon does is suck all the oxygen from the surrounding area ,so as the following troops, you could show up to a scene where all you buddies in the center are just ash and all your buddies at the edge are just dead with no marks on them. spooky for sure.
The nature of the explosive tends to put it own fire out quickly, limiting collateral property damge. Incidentally a specially designed fae would be ideal for dealing with people in caves and tunnels. I mean, as long as we're throwing the geneva convention and rule of law out the window anyway.....