The M-80 firecracker (also written as M80), is probably the firework that has had the most bullshit both thrown at it and talked about it.
The first thing to get out of the way is how powerful "they" are. If you went to any American public school, you undoubtedly heard at least one story about how the "bad kid" on the playground took an M-80 and blew up a car with it because, of course, an M-80 is Quarter/Half/Fifth/Third/One-and-four-fifths a "stick of dynamite". It's a sorry fact we have all been duped; even the weakest formulation of dynamite is more powerful than the composition in an M-80. M-80s contain "flash powder," which is not a high explosive and therefore you can't equate an M-80 with dynamite or any high explosive with any accuracy.
Sometimes these kids would even say that had some of these mythical ordnances in their possession and they were going to blow up your car with it. Well, sad to say, M-80's (and their close cousin the Cherry Bomb) have been illegal in the United States since 1966 under the "Child Protection Act". You can't make 'em, sell 'em or buy 'em anywhere in most of the "Free World" anymore. If you happen to find someone down a dark alley that claims to have "genuine" M-80's that they want to sell you they are either A) Lying, B) Lying, or C) a fence for a smuggler. Oh, sure, they may be some hand-made boom-boom; but do you think that's a mark in the PLUS or MINUS column?
A true, historically accurate M-80 is a firecracker used in the long past days of yore for military use as a "gunfire simulator". The original specification calls for a tube that is 1.5 inches long, 9/16th of an inch in diameter, with a fuse coming out the side rather than the end, and containing 45 grains of a specific pyrotechnic composition (which equates to just a little over 20 times the legal limit). This is probably where the "M" in "M-80" comes from -- "M" for "Military".
Sometimes you'll run across firework stands selling poppers with names like "M-60", "M-70", "M-88", "M-90" and so on. These are not kin of the M-80s save in visual appearance only. They may "look" like the original with the side-mounted fuse, but they are regular 50 milligram firecrackers, nothing more. However, variants with names like "M-100", "M-1000" and Quarterthumper are legitimate members of the M-80 family (and are, obviously, illegal in the US).
Reference Interests: http://www.atf.treas.gov/press/fy01press/062201fireworks.htm
Note: This information is from a North American Slant. Specificationally correct M-80's are still available in other countries around the world.