M-80s, also called Salutes, are an American class of large firecrackers, but unlike M-60s which are legitimate fireworks sold in the market today, M-80s are illegal in the United States.
By law, a firecracker sold for on-the-ground consumer use can contain no more than 50 milligrams of explosives. An M-80 has at least 3,000 mg.
Classified by the federal government as a dangerous explosive, M-80 has been banned for amateur use since 1966.
M-60s are regular 50 milligram firecrackers made to look like M-80s or "Silver Salutes," (another old type of firecracker now banned). These novelty firecrackers are legal to sell to the public, in states that allow firecrackers, because they contain no more than 50 milligrams of pyrotechnic content.
M-60s are nowhere near as potent as a true M-80. The names used for these ("M-60", "M-70", "M-88", "M-90" and so on) are basically meaningless marketing gimmicks.
For more info, visit FireworksLand.com.
If you plan to use fireworks, keep yourself as safe as possible with FireworksSafety.org's recommended safety tips.