Which drive to I select to completely erase my system's hard drive?
One of these icons represents the hard drive (the one with gobbledygook as a name), whereas the other represents a partition inside that hard drive. Usually there's a single partition per drive, but some power users may have reasons to create more than one. If you fall into the default case of a single partition, and you intend on staying that way -- or just want to erase your personal data and don't care how the drive will look like so long as your data's not in there -- then just go ahead and format the partition (the one for which you can start the deletion process). All of your data will be gone; maybe there's also a recovery partition hidden from view, which won't be affected by this process, but it won't contain any of your personal data.