Every week Catholics, Eastern Orthodox, and other Christians recite the Nicene Creed at the Mass (liturgy). The Nicene Creed is an ancient expression of the Christian faith that, in essence, affirms that the Father, Son, and Holy Spirit are distinct in some way, yet all equally God (in substance/essence). It was written in part against a heresy called Arianism, named after Arius of Alexander, a priest who denied Jesus was God. The end part about the Holy Spirit was added later in the 4th century, at the Council of Constantinople, to address those who denied the Holy Spirit was fully God.
Today, the debate seems kind of remote (Arius believed Jesus could be called “god” but not “true god”, and that the Son and Father were of a similar substance, as opposed to the same substance), but there are still those that would suggest Jesus is only a good man or even an angel (as the Jehovah’s Witnesses believe). The creed also contains information related to the historical life of Jesus, and is not simply metaphysical speculation (as is sometimes leveled against it).
For more detailed information, check out The Nicene Creed: Ancient Symbol of the Catholic Faith from our sister site, Ancient and Future Catholics.