You are not confused if you are able to answer following questions without any doubts
Q) Can we build a system which is highly fault-tolerant but not highly available?
Q) Can we build a system with low reliability and High availability?
Q) is reliability defined as MTBF or MTTR?
A system whose performance improves after adding hardware, proportionally to the capacity added, is said to be a scalable system.
Elasticity often refers to a system's ability to allocate additional resources in an autonomic manner
In other words, a scalable system allows you to add resources in order to handle more load, while an elastic system will add resources itself when the load increases.
Fault tolerance refers to a system's ability to continue operating, perhaps gracefully degrading in performance when components of the system fail. There is no exact measure to measure fault-tolerance of a system
Availability is a percentage of time that a system is actually operational and providing its intended service.
A = Uptime/(Uptime + Downtime)
Ai = MTBF/(MTBF+MTTR)
Where there are no single points of failure might be considered system as fault tolerant, but if application-level data migrations, software upgrades, or configuration changes take an hour or more of downtime to complete, then the system is not highly available.
In simple words, how long can a system stay up continuously?
More concrete definition is “reliability is the ability of a person or system to perform and maintain its functions in routine circumstances, as well as hostile or unexpected circumstances”
Reliability is often defined in terms of mean time between failures (MTBF). We can build a system with low-quality, not-so-reliable components and subsystems, and still achieve HA.
Durability of a system guarantees that stored data can't be lost.