According to the New Testament, elders are responsible for the primary leadership and oversight of a church. The function and role of an elder is well summarized by Alexander Strauch in his book Biblical Eldership: "Elders lead the church [1 Tim 5:17; Titus 1:7; 1 Peter 5:1-2], teach and preach the Word [1 Timothy 3:2; 2 Timothy 4:2; Titus 1:9], protect the church from false teachers [Acts 20:17, 28-31], exhort and admonish the saints in sound doctrine [1 Timothy 4:13; 2 Timothy 3:13-17; Titus 1:9], visit the sick and pray [James 5:14; Acts 20:35], and judge doctrinal issues [Acts 15:16]. In biblical terminology, elders shepherd, oversee, lead, and care for the local church" (16).
"Elder" and "pastor" are not two different offices. As John Piper argues in section five of the booklet "Biblical Eldership," they are simply two different words for the same office. He gives three reasons. First, in Acts 20:28, elders are encouraged in the "pastoral" duties of overseeing and shepherding. Second, in 1 Peter 5:1-2, elders are exhorted to "shepherd" the flock of God that is in their charge, which is the role of a pastor. Third, in Ephesians 4:11, the one time that the word pastor occurs in the NT, pastors are treated as one group with teachers. This suggests that the chief role of the pastor is to feed the flock through teaching, which is a primary role of elders (Titus 1:9). Hence, the NT seems to indicate that "pastor" is another name for "elder." An elder is a pastor, and a pastor is an elder.
Some have thought that the Bible speaks of a category of church leaders above elders/pastors, called "overseers." However, the biblical evidence indicates that "overseer" is simply another term for elder as well. Paul refers to the elders at Ephesus as "overseers" in his farewell sermon of Acts 20:17-35. Likewise, "overseer" in Titus 1:7 seems to be a synonym for the term "elder" used in verse 5. Most scholars now acknowledge this, as J.B. Lightfoot pointed out already in the 19th century: "It is a fact now generally recognized by theologians of all shades of opinion, that the language of the New Testament the same officer in the Church is called indifferently 'bishop [overseer]' (episkopos) and 'elder' or 'presbyter' (presbyteros)" (quoted in Strauch, 180).
John Piper summarizes the biblical use of the terms "elder," "pastor," and "overseer":
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.
Alexander Strauch, Biblical Eldership