Not all churches are called a parish. As far as the church hierarchy is concerned, the least ones are known as the sub-parish, and one parish heads a couple of sub-parishes. Every city of a particular state has a diocese, and parishes and the sub-parishes governed by them are all a part of that diocese. That quite answers your question. So all we now have to know is the difference that the churches have between them.
Sunday mass celebration:
The first preference for a Sunday mass is always given to the parish. If the parish priest can allocate time to celebrate mass in the sub-parish as well, then the sub-parish will have the celebration as well. If not, there are two options. The parishioners can either visit the parish for mass or can celebrate mass the previous evening. An evening mass on Saturday is considered equivalent to that of a Sunday mass. Most sub-parishes will have the mass celebration almost all Saturdays.
Holy Eucharistic Adoration:
The presence of the Holy Eucharist and the display of the tabernacle are usually found in the main parish. However, a tabernacle is present in every church except for if it is a small chapel or if the sub-parish is too small. Chapels are small places of adoration. Since the Holy Mass is not celebrated on a regular basis is places, as this the tabernacle is not found here.
Duties of the parish priest:
The priest of the parish can see no difference between a parish and a sub-parish. Sun-parishes came only to help people visit the church on a regular basis, and a parish was governing two or three small church so that too much of decentralised authority doesn’t exist. So a priest cannot show differences or treat his parishioners differently. But as far as his responsibilities are concerned a parish priest will have more task to accomplish in his parish, and he resides in a bungalow that is close to or inside the parish compound.
During big celebrations in the church, parishioners are required to collect themselves and come to the parish to take part in the celebration. On special occasions like Christmas, Easter and other celebrations it might not be possible for the priest to have multiple celebrations. So if the churches are located close by, which in most cases they are, can come together to host the event.
Cathedral – The Diocesan Church:
All these churches that are a part of the city come under the governance of one church which is called the cathedral. The bishop is the head of the district council of churches and is the head priest of the diocesan church.