The distinction between a parish and a congregation is the distinction between a territory and a group, and almost everything else about the two follows from it.
A parish has boundaries. Everyone inside them belongs to it whether they attend or not, whether they believe or not, and whether they know it or not. The priest posted there has a duty toward all of them, which historically meant that he could not decline to bury a parishioner he disliked or refuse to marry a couple whose attendance was poor. That obligation is the source of the parish's civil usefulness: because someone was required to record every baptism, marriage and burial within a defined area, parish registers became the only continuous vital records in much of Europe, and genealogy and historical demography as disciplines rest on them.
Catholic canon law makes the parish an office rather than an assignment. Its pastor has defined rights, a stability of tenure, and cannot simply be moved at a bishop's convenience, which is a real constraint on episcopal power and a frequent source of friction when dioceses close churches.
In England the parish became a unit of secular administration as well, responsible for poor relief, highways and constables until the nineteenth century transferred those functions to civil bodies. English parish boundaries are still drawn on maps for reasons that have nothing to do with religion, and civil parishes now exist that no church has ever served.
The territorial principle is weakening everywhere. Congregations shrink, people drive past three churches to reach the one whose music they prefer, and dioceses merge parishes into clusters served by one priest. What is being lost in the process is precisely the obligation toward the people who do not turn up, and that is a larger change than the administrative language of pastoral reorganisation suggests.