SMART (Self-Monitoring, Analysis and Reporting Technology) is a protocol
for passing information from a disk or SSD to the CPU. It's part of the ATA and SCSI standards and is based on work by IBM, Seagate and others done in the '90s.
The protocol has a consistent data structure able to report over 70 statistics, but what gets measured is up to the vendor. SMART looks at the trends in these and other measures to determine if the drive is headed for failure.
Backblaze started tracking SMART data
earlier this year on almost 40,000 drives to see if and how SMART could help predict drive failure. A drive failure is counted when the drive either won't power up, is bricked, or is showing (SMART) evidence of failing soon.