St. Patrick is credited with the explanation using a shamrock (3-leaved clover type of plant) as a tool. In basic terms, the shamrock has three small leaves all attached that makes up one whole. It can be used as a model to describe one God, but three persons (or personalities, if you would like). Each of them are all God, are all divine, yet distinct in the way we know or understand Him; hence, the Father, the Son, and the Holy Spirit.