The acquisition of US citizenship is often a product of permanent resident status. The duration involved for acquiring citizenship is contingent on the source of permanent residence.
Employment-based green card holders and most other permanent residents will acquire US citizenship after five years of USA residence. Permanent residence based on marriage would curtail the durational requirements to gain USA citizenship to three years.
Permanent residents enjoy most of the benefits afforded to citizens; however, their status is more fragile than that of a citizen. There are certain conditions bestowed on this status that may result in the loss of the right to reside in the USA. Citizens are not subject to such conditions and it is, therefore, prudent to apply for green card status once eligible to do so.