It's a common practise, but it's not a standard AFAIK. It's from latin mille which means 1000, so M=1000, and MM=1000000.
Unfortunately there is also a much more widely used international standard where k=1000 and M=1000000, so the M/MM forms are to say the least, confusing, and ought to be scrapped.
Is this just a standard/common practice in America? I have yet to see it used anywhere else but then I am primarily exposed to UK/European news sources.
When discussing certain quantities, including money, it's common to use MM such as $100MM.
http://en.wikipedia.org/wiki/SI_prefix#Similar_symbols_in_ab...