Synod is a procedural word and the only useful question about any particular one is what it is empowered to do. The answers range from making law to offering advice that may be disregarded.
At the strong end, the General Synod of the Church of England legislates. Its measures, once approved by both Houses of Parliament and given royal assent, are the law of England. No other church body in the country has that power, and it exists because the Church of England is established. The synod has three houses, bishops, clergy and laity, and contested measures require a majority in each, which is why the ordination of women to the episcopate failed in 2012 on the house of laity alone and passed in 2014.
Orthodox holy synods elect bishops, try them, and settle doctrinal and liturgical questions for their own autocephalous church. There is no higher body, since there is no centre, and the granting of autocephaly to a new church is itself a synodal act whose validity other churches may dispute, as happened over Ukraine in 2019.
Presbyterian polity uses synod as one rung of a graded court system, between presbytery and general assembly, with defined original and appellate jurisdiction. These are courts in a real sense, with procedure, evidence and appeal.
At the weak end sits the Synod of Bishops of the Catholic Church, established in 1965. It is consultative. It advises the pope, who then issues a document of his own; it cannot legislate, and its recommendations bind nobody. Recent assemblies have broadened who attends and who votes, including laypeople and women for the first time, which changed the composition without changing the constitutional position, and much reporting has treated the two as the same thing.
The Reformation-era synods produced confessional documents that still operate as legal standards in the churches descended from them, which is a reminder that the decisions of a synod can long outlive the assembly that made them.