Modalism included several heresies in the A.D. 100s and 200s which insisted that there is only one person in God, but that God manifests Himself in three different modes or ways: Father (creator), Son (redeemer) and Holy Spirit (sanctifier). This differs from the Trinitarian view held by the majority of Christians, which teaches that the Father, Son, and Holy Spirit are three distinct Persons, yet one God.
Modalism continues to this day. One of the most prominent exponents of modalism (though he refuses to admit such) is T.D. Jakes.