DiskFailure has a single purpose, to warn you about a potential or imminent hard disk failure before the disk is rendered nonfunctional/data is lost.
It will warn you trough alert dialogs, growl notifications and the menubar icon, it also features a detailed list of your drives and the monitored parameters for each.
It also keeps track of these parameters trough all it's lifetime to provide a cumulative slate of past history for any storage device connected.
Any drive is considered nearing imminent failure when IO errors are detected, temperature*, bad blocks, drive starts and load cycles exceed certain thresholds.
HDD and SSD drives are handled differently but all are supported if connected either trough ATA/SATA USB/FireWire** (ThunderBolt drives are not supported yet).