No, it certainly won't. It will only erase your "stuff" if you use Disk Utility, select the disk, and click the erase button. It will warn you that it is going to do that.
The only way to revert to Snow Leopard is to erase the disk though. Even Time Machine will erase the disk, before restoring an earlier backup.
The reason for all this is that you cannot install a version of OS X earlier than the one installed without erasing the disk first. If you have Snow Leopard and want to try Mountain Lion, I really recommend you use Time Machine, or clone the disk first. This is the only way to effectively "change your mind" if it turns out you don't like Mountain Lion, and want to turn back the clock.
can I revert back to mac os 10.6 after upgrading to 10.8?
Yes you can, however it is A LOT easier if you make a 10.6 TimeMachine (disconnect it) and/or (best) a 10.6 Carbon Copy Cloner bootable clone. (disconnect it too)
TimeMachine drives are not bootable, so you will require your 10.6 install disk (slow) to be perfectly functional to boot and perform the ENTIRE drive erasure required to restore the 10.6 TimeMachine copy (slow) to the internal drive.
With a 10.6 bootable clone on a external drive, it's hold the option key bootable, so your up in seconds on the 10.6 clone and can be erasing the entire internal drive while surfing the web.
Also if your 10.6 disk is not working, the clone is, so it's another bootable backup.
The other more complicated method to restore 10.6 on a machine that was upgraded to 10.7 or 10.8 is to extract all one's files into a neutral state on a external storage drive. (not TiThen use the 10.6 disk to wipe everything and reinstall 10.6 fresh, then programs then files.
10.7 and 10.8 changes things in the User/Library, so you can't just restore from TimeMachine backwards.
Also newer versions of programs from Apple changes their support files, like iTunes Library and iPhoto Library, so it's a lot more work extracting your original mp3's and jpg's for reimportation into 10.6 versions of those programs.