Backing up is one of the most important things you can do on a Mac, and thankfully, it's a pretty trivial affair with Time Machine built right in. Of course, when you're looking at what needs backing up, people often overlook what doesn't need backing up (or isn't, but should be).
A good example of this would be a rented media file, like a TV show rented from iTunes. It's only valid for 48 hours, so even if you do back it up, by the time you've restored your machine after a system failure, it's unlikely to play anyway. In essence, it's wasted space.
Thankfully, OS X has a built-in system of defining what should and shouldn't be backed up using metadata, and for the most part, it happens under your nose without you even noticing. There are times when it would be useful to see what's marked for backup and what's not, however, and that's where we turn to Terminal.