The word synod which comes from two Greek words syn (together) and hodos (road) means “a coming together” or “a meeting.” A synod is a council of bishops, as in the case of the Synod of Bishops created by Pope Paul VI in 1965, which meets periodically at the invitation of the reigning pontiff. Unlike an ecumenical council where every Catholic bishop in the world is invited, a synod is open only to a representation of bishops from each country or national episcopal conference. It has no legislative authority of its own but it advises the pope on current issues and concerns.
The pope sets the agenda, summons, suspends, and dissolves the synod. He can preside himself or appoint a president from the member bishops. There have been twenty-one synods since its creation (1965). Before each begins, a Lineamenta (outline) is given to the bishops to set the tone and scope of the upcoming synod. Suggestions from the bishops on possible discussion topics related to the outline are sent to Rome. Once convened, the bishops are given an Instrumentum laboris (working copy) outline based on the Lineamenta and the suggestions it inspired.
A Metropolitan Archbishop can convene a provincial synod where the bishops from that area gather. For example, the Cardinal Metropolitan of Philadelphia could convene a provincial synod which would entail the Archdiocese of Philadelphia and the suffragan sees of Erie, Pittsburgh, Greensburg, Altoona-Johnstown, Harrisburg, Scranton, and Allentown.
National synods are less common, and the United States has had only three so far. The first three Plenary Councils of Baltimore took place in 1852, 1866, and 1884, and from them came the famous Baltimore Catechism which was the first national catechism for the USA.
A Diocesan Synod is a little different in that there is usually only one bishop, the local Ordinary, or a few if there are auxiliary or retired bishops in the diocese. Most of the synodal participants in a diocesan synod would be representatives of the local clergy, religious, and lay faithful. The diocesan bishop is the only one who has a vote but the very reason he convenes a diocesan synod is to receive counsel and advice from his priests and his lay faithful. The synodal decrees which the bishop approves become diocesan (local) law. They must, of course, be in conformity to the universal canon law of the church and cannot contradict any article of the faith nor can they conflict with the general liturgical laws of the Roman Missal (for the Mass) or Roman Ritual (for the other Sacraments).