These terms define distinct theological frameworks regarding the nature of the Godhead.
Unitarianism is the strict monotheistic belief that God exists as only one single divine Person (the position held by Judaism, Islam, and certain non-mainstream Christian groups).
Binitarianism is the belief that the Godhead is comprised of exactly two divine Persons, typically the Father and the Son, acting in complete unity.
Trinitarianism is the orthodox, mainstream Christian doctrine defining God as one absolute divine Being who exists eternally and simultaneously as three co-equal, co-eternal Persons: the Father, the Son, and the Holy Spirit.