I do not have an author's name...but I have used this many times because the subject about the Trinity comes up so much. I keep this close by to share with people.
In the simplest of terms, Christians believe:
There is only one God, and this one God is in three Persons.
The three Persons are:
God the Father
God the Son (Jesus Christ)
God the Holy Spirit (also called the Holy Ghost)
The Father, Son, and Holy Spirit are all God, but they are not three names for the same Person.
The Persons are distinct:
The Father is not the Son.
The Son is not the Holy Spirit.
The Holy Spirit is not the Father.
God is one absolutely perfect divine Being in three Persons. His being is what God is, in relation to the universe he created. The three are called Persons because they relate to one another in personal ways.
When Christians talk about believing in one God in three Persons (the Trinity), they do NOT mean:
1 God in 3 Gods, or
3 Persons in 1 Person, or
1 Person in 3 Gods
Rather, they mean:
1 God in 3 Persons
Therefore,
The Father is God---the first Person of the Trinity.
The Son is God---the second Person of the Trinity.
The Holy Spirit is God---the third Person of the Trinity. (The title "Holy Ghost" is an older English expression for "Holy Spirit." Each is an acceptable translation of the phrase in the Bible.)