At its heart, tier (as it is called in Second Life) is the capacity to own land. This capacity is paid for in a number of steps (or tiers, hence the name). Each ownership capacity tier is double the area of the previous tier.
Tier is a ubiquitous concept in Second Life, but a frequently misunderstood or an incompletely understood one. Understanding land-tier in Second Life is important, whether you are owning, renting, or part of a group that owns land, so read on, and we'll tell you all about it.