Recently got Mountain Lion. Before installing, in case I wanted to do a clean install, I followed the steps on one of the many tutorials you can find online, on how to make Mountain Lion bootable on a USB drive. After finding the InstallESD.dmg (within show package contents), I was curious so I clicked the install icon within the .dmg to see if my computer would upgrade (from Snow Leopard 10.6.8). Once it did its thing, I was running Mountain Lion, with my old setup (i.e. upgraded). So I was wondering, what is the real difference between upgrading through the InstallESD.dmg and the actual app downloaded from the App Store? Never really could find an answer anywhere. It seems Apple has changed things a bit, as I don't recall anything like that since the last OS I got was Snow Leopard. Thanks! Hope my question makes sense.