The Reformed churches are a family of Christian, Protestant denominations whose doctrine is described in detail by the Reformed confessions. In the OPC, we subscribe to the Westminster Confession of Faith, and the Larger and Shorter Catechisms, produced by the Westminster Assembly, 1643-1648.
Other Reformed confessions include the Three Forms of Unity: The Belgic Confession, the Heidelberg Catechism, and the Canons of Dort
(The linked Three Forms of Unity are versions used by the Canadian and American Reformed Churches. They are in PDF format.)
For an overview of the history, doctrines and ministry of the OPC, the denomination publishes a helpful booklet that is available online called What is the Orthodox Presbyterian Church?