Reinstall will not wipe your data or installed applications, it will only install the OS files.
However as a standard precaution, you should backup your data before you reinstall, for the 'just in case' senario.
I would be asking first, what is the reason that you want to reinstall the OS, and what issue are you trying to resolve.
I just want to reinstall Mac OS X 10.8 Mountain Lion because some apps (Safari, Mac App Store etc...) took quite some time to load every time I opened them.
A few days ago, I went to my local Apple Authorised Reseller and saw that the entry level MacBook Pro that they had on display loaded Safari and many other apps really quickly. I wondered why this was so, when mine took longer to load. I asked them if they had altered it or something, and they said that they hadn't upgraded it and that it was in factory condition. I told them about my problem and they said it might be a glitch in the software while I downloaded the upgrade. So this is why I want to reinstall Mountain Lion.
That's cool, but just remember, that depending on a computer and it's configuration, application load times etc all run a little differently. (SSD drives are much faster at loading applications than traditional HDDs. Also, if you hard drive is quite full, it can run slower.)
Before I would proceed with a full reinstall the OS, I would run Disk Utility in the Applications/Utilities folder and repair disk permissions.
To do so, I would reboot my computer, make sure no applications are running, run DU, reboot again and then see if the performance has improved.
If things seem to be running slow, open up activity monitor
and set the dropdown menu to Show All Processes. Then
click the CPU column title to sort for desending CPU usage
and see if there is some app using lots of CPU.
In Activity Monitor, you can also check disk activity and network
activity to see if there is something excessive going on there.
Also, if you have any antivirus app installed and running turn it off.
These are notorious for creating system slow downs. If you really
want an antivirus app, turn off background operation and only
run if you suspect an issue.