Tier 1 – no redundancy
Tier 2 – some redundancy but still single points of failure
Tier 3 – concurrently maintainable – ie you can power anything down for maintenance and there is enough redundancy to keep things running (colloquially, and not always accurately known as N+1).
Tier 4 – double everything – sufficient redundancy is built in such that even after powering something down for maintenance, there is still redundancy in the system (colloquially 2N, actually at least 2(N+1)).
Ruthlessly stolen from here.