SM,
Very briefly, MJ-12 was a group of distinguished scientists, military and intelligence officials, established by President Harry Truman (or alternately, President Eisenhower, depends who's advancing the conspiracy) to control the recovery of UFOs. It also pertains to a set of documents that allegedly involves a working relationship or 'treaty' if you will with alien races that gives the US the right to reverse engineer alien technology and in exchange grants the aliens unrestricted freedom to 'abduct' as many of its citizens as they needed for their research.
Although still touted by Stanton Friedman as being credible, (probably because he has written 2 books on the subject), the documents themselves have been proven to be fraudulent, and the committee an elaborate fabrication. There's an excellent expose written by Philip Klass
HERE which details much of how the 'secret' originated. There's also quite a bit of information that can be found on the
SAUCER SMEAR website, use the search engine on the site for articles on 'Majestic 12'. If you can find the time to read them, you might find all the issues listed on the main index interesting and often humorous, if not extraordinarily revealing.