[Orthodoxy]

This is how I currently understand it....

A simplified explanation of modalism might be like this:

There is only one "person" in the Godhead. This person has worn different "masks" throughout redemptive history. He revealed himself as Father in creation, Son in redemption, and Holy Spirit at Pentecost.

A simplified explanation of classical Trinitarianism might be:

There are three distinct, co-equal, co-eternal, self-conscious "persons" within the Godhead. The Father is God, the Son is God, and the Spirit is God. However, the Father is not the Son, the Father is not the Spirit, and the Son is not the Spirit.

It's hard for me to see how, according to modalism, the Father sent the Son to die on the cross (because that would require two persons). I guess they would say that the divine nature sent the human nature to earth....I really don't know how Oneness adherents would explain the Incarnation.