MTBF (mean time between failures) is a measure of how reliable a hardware product or component is. For most components, the measure is typically in thousands or even tens of thousands of hours between failures. For example, a hard disk drive may have a mean time between failures of 300,000 hours.
Software as a Service (SaaS) is a model of software distribution whereby software applications deployed as a hosted service and accessed over the Internet via a standard web browser.