Committees are headed by an elder who is currently serving on the session of St. Mark. These committees take care of the work of the church, where members and elders take care of the details of service and worship.
From the PCUSA website:
A major contributor to Reformed theology was John Calvin, who converted from Roman Catholicism after training for the priesthood and in the law. In exile in Geneva, Switzerland, Calvin developed the presbyterian pattern of church government, which vests governing authority primarily in elected laypersons known as elders. The word presbyterian comes from the Greek word for elder.
Elders are chosen by the people. Together with ministers of the Word and Sacrament, they exercise leadership, government, and discipline and have responsibilities for the life of a particular church as well as the church at large, including ecumenical relationships. They shall serve faithfully as members of the session. (Book of Order, G-10.0102) When elected commissioners to higher governing bodies, elders participate and vote with the same authority as ministers of the Word and Sacrament, and they are eligible for any office. (Book of Order, G-6.0302)
The body of elders elected to govern a particular congregation is called a session. They are elected by the congregation and in one sense are representatives of the other members of the congregation. On the other hand, their primary charge is to seek to discover and represent the will of Christ as they govern. Presbyterian elders are both elected and ordained. Through ordination they are officially set apart for service. They retain their ordination beyond their term in office. Ministers who serve the congregation are also part of the session. The session is the smallest, most local governing body.