M7 is the collection of top B-schools whose deans meet every so often to discuss their curricula and the future of MBA education. The M7 schools are:
-HBS
-Wharton
-Columbia
-Kellogg
-Chicago
-Stanford
-MIT Sloan
More funny but true comments below:
Here's a quick guide:
If someone describes their school as the top ranked school, they go to HBS or Stanford
If someone says they go to a "Top 3" school, they go to Wharton
If someone says they go to a "Top 5" school, its either Chicago, Kellogg, or MIT Sloan
If someone says they're in the "M7", they go to Columbia
If someone says "Top 10", they mean Tuck, Ross, Haas, or <insert whatever school you insist belongs in the top ten>
Have a fun!