Each of our leaders serve different capacities and their roles serve different functions according to the responsibilities given to them in the Bible.
Elders are an important part of any congregation. So important that Paul appointed elders and had elders appointed in every congregation (Titus 1.5; Acts 14.23). In the New Testament, elders are also called shepherds (Eph 4.11; 1 Pet 5.1-2), which most people know as pastors, overseers (Acts 20.28; 1 Tim 3.2), which many know as bishops, and simply leaders (Heb 13.17).
Elders serve a very important function in the Lord's Church as is indicated by the different terms used for them. "Elder" conveys wisdom and guidance. "Shepherd," like the one who cares for physical sheep, conveys the idea of one who both protects against foreign attacks and keeps the sheep from straying. "Overseer" conveys the idea of managing and providing. The greatest of all responsibilities comes with the word "leader" in Hebrews 13.17, "as those who will give an account for your souls." Elders are the spiritual leaders and caretakers of the congregation.
Deacons are servants of the church. Deacons are described in 1 Tim 3.8-12. Deacons got their start as a group of seven men who were responsible for delivering food to widows in the first century in Acts 6.1-6. The purpose of the deacon comes from Acts 6.3, so that the apostles did not need to neglect spreading the word of God to "wait tables." Deacons serve an invaluable service to the congregation to care for the physical needs of a congregation so that the eldership can care for the spiritual needs of the congregation as they have been appointed.
Ministers have no authority of their own. The only authority that preachers have is the Word of God as their function is to spread the gospel to those have need of it (Rm 10.14). Preachers also teach, evangelize, and minister to the needs of the congregation.