The Reformed Churches have a number of governing bodies. Their relation to each other is marked by a careful graduation. They are known as consistory, classis, and synod. The consistory consists of the minister and the elders of the local church; the classis, of one minister and one elder of each local church within a certain district; and the synod, of an equal number of ministers and elders from each classis.
a. The government of the local church. The government of the local church is of a representative character. The minister and the elders, chosen by the people, form a council or consistory for the government of the church, Acts 14:23; 20:17; Tit. 1:5. While the elders are chosen by the people, they do not receive their authority from the people, but directly from Jesus Christ, the Lord of the Church. every local church is a complete church, fully equipped to rule its own affairs. But since it affiliates with other churches on the basis of a common agreement, it is not entirely independent. The Church Order serves to guard the rights and interests of the local church, but also the collective rights and interests of the affiliated churches.
b. The Major Assemblies. When local churches affiliate to give greater expression to the unity of the Church, major assemblies, such as classes and synods become necessary. The council of Jerusalem, described in Acts 15, partook of the nature of a major assembly. The immediate representatives of the people, who form the consistories, are themselves represented by a limited number in classes, and these in turn are represented in synods. Ecclesiastical assemblies should naturally deal only with church matters, matters of doctrine and morals, of church government and discipline. But even so major assemblies must limit themselves to matters which as to their nature belong to the province of a minor assembly, but for some reason cannot be settled there; and matters which as to their nature belong to the province of a major assembly, because they pertain to the churches in general. The decisions on major assemblies are not merely advisory, but authoritative, unless they are explicitly declared to be only advisory.
[From Berkhof Summary of Christian Doctrine]