Modalism is the heresy that says that God exists in one person, who interacts with man in three different "modes" or "roles". IOW, Father, Son, and Holy Spirit are not three persons, but descriptive of three ways that a (unitarian) God acts or reveals himself.
Orthodoxy says, OTOH, that Father, Son, and Holy Spirit are three distinct persons sharing one divine nature.
The "United Pentacostals" are modern-day modalists.