I'd like to be able to use my current system (Tiger, OS 10.4.11) and Snow Leopard at the same time. I would like to compare them before I commit to one or the other.

I just ordered Snow Leopard (OS 10.6.3) from Apple and have the install disc. I never partitioned a drive before or used two OSes at once.

How do I install Snow Leopard so that I can use it along with Tiger rather than replace Tiger on the same hard drive on my imac?

I have a backup drive with a OS 10.4.11 clone.

You can not run both OSes at the same time on the same Mac, but you can have them installed on separate volumes (partitions or disks) at the same time and run them consecutively.

So what you'd need is a(nother) partition or disk that doesn't already hold a copy of Mac OS X, or which you can overwrite with Snow Leopard. Certain utilities allow you to partition existing volumes that already contain data (like your iMac's HD), but because there's always the risk of data loss it's best to start with a fresh partition/disk. Apart from that, such utilities may not run on Tiger, which would leave you only the extra disk option. (But see Stellar Partition Manager, with which I'm not familiar.)