Using Time Machine, make a backup just before making a change to your system and note the time. You can then choose that particular backup if you need to restore your HD.
Another method is to create a partition the same size or larger than your internal drive, start up using the Recovery HD partition (if you have Lion), or your system restore discs, and use Disk Utility to create an image backup of your internal HD (with Snow Leopard, or earlier versions, make sure to check "erase destination..." to enable block copy).
When doing the initial partitioning of your external drive, make sure to set the table type to GUID ("Options..." button on the Partiton tab). This will allow you to boot off that drive, if necessary (for instance, if you make an image backup using Restore, it will be bootable).
Edit: Lion actually does have built-in local backups which are made automatically and sync with Time Machine.
One reason it doesn't have a specific, built-in, "snapshot" ability (if you exclude Time Machine) is the way Mac apps are installed. Generally, it's simply a matter of trashing the app itself. Any remaining parts are usually in the form of preference files, which are small, unique to that app, and can be eiher ignored, or simply trashed. Apps don't spread out into the system via registry, dlls, etc., the way Windows apps generally do (though, that has improved over the years). You'll find more detailed info on differences between how OS-X and Windows handles things here, and probably those others sites as well. It's not something inherent to the latest version of OS-X, it's been done this way all along.
