An Anabaptist is a member of one of the many Anabaptist Christian sects which emerged during the Radical Reformation in the 1600s or even later. Some examples of churches which are considered Anabaptist include the Amish, Hutterites, Baptists, Mennonites, Church of the Brethren, Quakers, and Brethren in Christ. As you can gather from this list, Anabaptism takes a number of forms, and it is an incredibly diverse branch within the larger theological family of Christianity.
The defining mark of Anabaptism is a rejection of the concept of infant baptism. Anabaptists believe that infants cannot be held accountable for sin, because they have no knowledge of good or evil, and so they do not baptize their infants or honor infant baptisms. Instead, an Anabaptist believes that baptism is a confession of faith, and Anabaptists practice believer's baptism, in which people choose baptism as an expression of their religious faith.
The Anabaptists hold several other traits in common. They consider the Bible to be the ultimate religious authority, rejecting the authority of Rome over the Christian faith, and many also take their inspiration from the Early Church, attempting to live as Jesus and His contemporaries did. The rejection of infant baptism is also a reflection of early Church values; Jesus Himself, after all, was baptized as an adult by John the Baptist.
Many Anabaptist sects also place an emphasis on living a simple life. It is common for members of an Anabaptist sect to work together on charitable projects as well, and many Anabaptists feel that charity is an important expression of Christian values. Some Anabaptist churches are also pacifist, and on occasion quite radical, arguing that Christ was radical and that they should follow in His footsteps.
The Reformation was a period of intense upheaval and struggle in Europe, and the Anabaptists were only one among an assortment of splintering Christian sects which rebelled against Rome and attempted to redefine faith. Many Anabaptists believed in the separation of Church and State, among other things, and they were considered radicals by their contemporaries. The term “Anabaptist” was actually originally a pejorative, meaning “to baptize again.”
In the United States especially, Anabaptism is alive and well, with numerous churches across North America expressing Anabaptist values. This is probably related to the mass exodus of Anabaptists from Europe in the wake of the Reformation; some thought that they could create their own religious communities in the New World once they were free of persecution. The United States also houses some very conservative Anabaptist sects, such as the Amish and Old Order Mennonites.