Hard drive space

I have been off-loading files to external hard drives to prepare moving from a 750 GB MacBook Pro to a 500 GB MBP. When I check Get Info on the hard drive in the Finder, it tells me that 569 GB are available. When I check "storage" from About This Mac of the Apple menu, it says that I have 100 GB free. Oddly, 469 GB are listed as "Backups". What are these "Backups", where are they, and why the discrepancy?

15-inch, late 2011 MacBook Pro running 10.10.3