There's "Upgrade", which just does a normal upgrade install and changes whatever files it needs to so that your previous OSX installation is then, in fact, a Leopard installation. (Magic!) If you have no misgivings about wanting a 'clean' system and don't have any reason to believe that an Upgrade install will cause problems, then this is your best bet.
Then there's "Archive and Install", which will take all of your current system files and stick them all in '/Previous Systems' right before it does a completely clean install of the Leopard on your disk. You also have the option with this to "Preserve Users and Network Settings" which will automatically migrate your users and network settings (obviously) over to the new Leopard installation so you don't have to mess with the tedious method of doing it manually. For people that like a 'clean' install but don't want to both with the next option...
Finally, "Erase and Install." Very simply it reformats your selected partition and install a perfectly clean and brand-spanking-new copy of Leopard on the fresh partition. Obviously if you had files you wanted to keep (like your Users folder), you would want to somehow back these up prior to doing this. For people who love taking the hard route and can't stand "dirty systems." Or if you're just really paranoid.
Have fun!