This is a good explanation
History
The earliest organization of the Christian churches in Palestine was similar to that of Jewish synagogues, who were governed by a council of elders (presbyteroi). In Acts 11:30 and 15:22, we see this collegiate system of government in Jerusalem, and in Acts 14:23, the Apostle Paul ordains elders in the churches he founded. Initially, these presbyters were apparently identical with the overseers (episkopoi, i.e., bishops), as such passages as Acts 20:17 and Titus 1:5,7 indicate, and the terms were interchangeable.
Shortly after the New Testament period, with the death of the Apostles, there was a differentiation in the usage of the synonymous terms, giving rise to the appearance of two distinct offices, bishop and presbyter. The bishop was understood mainly as the president of the council of presbyters, and so the bishop came to be distinguished both in honor and in prerogative from the presbyters, who were seen as deriving their authority by means of delegation from the bishop. The distinction between presbyter and bishop is made fairly soon after the Apostolic period, as is seen in the 2nd century writings of St. Ignatius of Antioch, who uses the terms consistently and clearly to refer to two different offices (along with deacon).
Initially, each local congregation in the Church had its own bishop. Eventually, as the Church grew, individual congregations no longer were served directly by a bishop. The bishop in a large city would appoint a presbyter to pastor the flock in each congregation, acting as his delegate.