Some users like to run clean installs of major operating system upgrades. This literally means backing up your computer or computers, wiping them clean and installing the new OS. It’s a pain, and it’s quite a chore, but it can work out quite nicely if you have the patience.
MacWorld has a breakdown of when or why it may or may not be ideal in your situation. They say that in most cases, upgrading directly to Mountain Lion should work just fine.
However, there are situations where a clean install may be recommended, such as “funky” partitions or other things users may have set up. That said, users who set partitions like that normally have a good sense of how to use their machines and can do much of this on their own.
Clean installs are also recommended for users that have been using their OS for a while and have installed and uninstalled lots of applications, etc.
You can read a more in-depth breakdown of all that at MacWorld’s website: Link