What's NOT in the 'Home' folder on a Mac? How can I make sure I'm backing everything up?

I’m backing up my laptop to an external LaCie HD. It automatically backs up the Home folder, which is mine as the only User on the computer. I know the Home folder leaves out Applications and Systems things… But I notice a big discrepancy in size between what’s being backed up and the total files on my computer not including applications.

I noticed that files in my HD folder weren’t there in the Home folder, so I moved them to Documents, which is. But there’s still a 15GB difference between the Home folder (as appearing in the backup software) and the size of my computer hard drive minus space available and applications (in other words, space used). So what’s the 15GB of stuff that’s not getting backed up? Where should I look? Or am I missing something? I remember hearing that the Systems take up around 10GB, although the System folder only shows as 2GB… but could that explain it?

Just want to be sure that if my computer dies I’ll lose as little as possible!