If you haven't updated to Lion yet, don't be in a rush. If you have, this might come as a bit of a shock but Apple has removed Rosetta PowerPC. Which means most of your old applications and games may not work. These apps will now have a white slashed circle on it thus officially declared by Apple to be dead. Another way to check is by right clicking on the app in Finder and clicking on "get-info" or pressing Command-I after selecting the icon in Finder. In the kind section it will say either "Application ( Intel, Universal or PowerPC ). The first two will work with Lion the last will not.
So how to keep Lion and Snow Leopard.
Simple: Upgrade to Lion and Install Snow Leopard on Parallels Desktop or VMware Fusion
Parallels Dekstop and VMware Fusion are crossover applications which allow you to run Windows ( or any other Operating System ) on your Mac without having to reboot your Mac.