A tonsure is the cutting of hair, and is a part of several sacraments of initiation. Today in Eastern Orthodoxy, there are three types of tonsure: baptismal, monastic, and clerical. It is an offering of self to God.
It always consists of the cutting of four locks of hair in a cruciform pattern: at the front of head as the celebrant says "In the Name of the Father," at the back of head at the words "and the Son," and on either side of the head at the words "and the Holy Spirit." In all cases, the hair is allowed to grow back; the tonsure as such is not adopted as a hairstyle. The hair which has been cut is burnt in the censer.
Monastic tonsure (of which there are three grades: Rassophore, Stavrophore, and the Great Schema) is the rite of initiation into the monastic state.
Clerical tonsure is done prior to the ordination to the rank of reader. This has lead to the common usage that one is "tonsured a reader," although technically the rite of tonsure occurs prior to the actual ordination by laying on of hands.