System
A system is composed of a collection of interacting components. A component may itself be a system, or it may be just a singular component. Components are the result of system decomposition chiefly motivated to aid in the partitioning of complex systems for either technical, or very often, for organizational or business reasons. Decomposition of systems into components is a recursive exercise. All components are typically delineated by the careful specification of their inputs and outputs and a component that is not decomposed further is called an atomic component.
Service
A system provides one or more services to its consumers. A service is the output of a system that meets the specification for which the system was devised, or which agrees with what system users has perceived the correct values to be.
Failure
A failure in a system occurs when the consumer (human or non-human) of a service is affected by the fact that the system has not delivered the expected service. Failures are incorrect results with respect to a specification or unexpected behavior perceived by the consumer or user of a service. The cause of a failure is said to be a fault.
Mean time to failure (MTTF)
Hardware reliability can be predicted by statistically analyzing historical data. The longer a component operates, the more it is likely to fail due to aging. The mean time to failure of a component is just that: a statistical forecast to measure the average time between failures with the modeling assumption that the failed system is not repaired. The greater the MTTF of a component, the less likely it is to fail. You can use the MTTF of a component (if it is known) as useful information for establishing preventative maintenance procedures and its value for an overall system can be improved by carefully selecting your hardware and software.
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