The New Testament only refers to the office of pastor one time (Ephesians 4:11). It is a functional description of the role of elder stressing the care and feeding of the church as God's flock, just as "bishop/overseer" is a functional description of the role of elder stressing the governing or oversight of the church. We may conclude therefore that "pastor" and "elder" and "bishop/overseer" refer in the New Testament to the same office. This office stands alongside "deacon" in Philippians 1:1 and 1 Timothy 3:1-13 in such a way as to show that the two abiding officers instituted by the New Testament are elder and deacon.
The function of elders may be summed up under two heads: teaching and governing. They are the doctrinal guardians of the flock and the overseers of the life of the church responsible to God for the feeding and care and ministry of the people.
Source: John Piper