I'd like to show the Launcher when I am on my desktop and hide it when an application is running. How can I personalize it?
You can autohide Launcher..
Desktop Screen ==> RightClick ==>Change Desktop Background => Behaviour (Tab) ==> Autohide [ON]
I'm not sure if it is possible to do exactly what you are asking as Unity is designed to look the same on every system (good promotion tactic), however you can get the unofficial 'Unity Tweak Tool' on a 12.10 PPA:
Press Ctrl + Alt + T on your keyboard and then run the following commands in order:
or you can upgrade to the Ubuntu 13.04 Release Candidate by pressing Ctrl + Alt + T on your keyboard and typing in:
and then getting the tool with:
Unfortunately the tool isn't available in Ubuntu 12.04.2 LTS due to some technical difficulties.
If the tool doesn't do what you want it to, try learning the C/Vala/C++(/NUX) programming languages and then changing Unity so it does what you want. That is half the point of open-source after all.