Over the centuries, the Church has used several Creeds to teach true doctrine to converts and to clarify certain teachings (dogmas) that were disputed. Many of the Protestant and Orthodox denominations still use the same Creeds. Interestingly, most of the denominations that still use the Creeds even keep use of the credal phrase "I believe in one, holy, Catholic, and Apostolic church," even though they no longer consider themselves Catholic, at least with a big "C."Others have changed "Catholic" to "Christian."
The word Creed comes from the Latin "Credo," which means "I believe." The meaning of the words "I believe" includes intellectual agreement. In addition "I believe" in this context implies that I not only agree in my mind, but that I also trust the source. Catholic believers believe that the source for the Catholic Creeds ultimately is Jesus Christ, who through the Holy Spirit created the Scriptures and guides the doctrinal definitions of His Church, which is His Body on this earth.
Many writers have pointed out that when we say "Credo" or "I believe," the roots of the word "Credo" carry even more meaning, not only agreement and trust, but also both love and commitment.
This means that the fullest implication of the words "I believe" when a Catholic says the Creed is something like "I give my heart-felt commitment to these truths because I not only agree with them but I also trust and love the Creator who revealed them to us and the Catholic Church that defined them."
Catholics are mostly familiar with two Creeds that are recited during Masses: The Apostles Creed, and the Nicene Creed. Less well-known is the Athanasian Creed.
The Apostles Creed, was also called the Symbolum Apostolorum (Symbol of the Apostles). It was a kind of passphrase among Christians in the early Church, like a password given out by a military leader to the troops. You would know that someone was a Christian, if that person knew the Creed.
The purpose of a creed is to act as a yardstick of correct belief, or orthodoxy. The creeds of Christianity have been drawn up at times of conflict about doctrine: acceptance or rejection of a creed served to distinguish believers and deniers of a particular doctrine or set of doctrines. For that reason a creed was called in Greek a σύμβολον (Eng. sumbolon), a word that meant half of a broken object which, when placed together with the other half, verified the bearer's identity.
Tradition with a small "t" divided the statements of the Apostles Creed into twelve articles, it was believed that each of the Apostles contributed one of its articles:
1. I believe in God, the Father almighty, creator of heaven and earth.
2. I believe in Jesus Christ, his only Son, our Lord.
3. He was conceived by the power of the Holy Spirit and born of the Virgin Mary.
4. He suffered under Pontius Pilate, was crucified, died, and was buried.
5. He descended to the dead. On the third day he rose again.
6. He ascended into heaven and is seated at the right hand of the Father.
7. He will come again to judge the living and the dead.
8. I believe in the Holy Spirit,
9. the holy catholic Church, the communion of saints,
10. the forgiveness of sins,
11. the resurrection of the body,
12. and life everlasting.
The twelve articles of the Apostles' Creed were put as questions to candidates for baptism. "Do you believe in God, the Father almighty, creator of heaven and earth?" And so on.
The Nicene Creed was created in response to heresies that denied that Jesus Christ was God. To combat those errors, articles were added to affirm the correct doctrine that Jesus is "God from God, Light from Light, true God from true God, begotten, not made, one in Being with the Father ... ."
Although it is now commonly referred to as the Nicene Creed, it is more accurately referred to as the "Niceno-Constantinopolitan Creed", because the creed defined at the Nicene Council in A.D. 325 was revised in the First Council of Constantinople in 381.
The Athanasian Creed is the first creed that stated the equality of the three persons of the Trinity, and it includes anathemas, or condemnations, of those who disagree with the truths expressed in that Creed. (Anathemas were also part of the original Nicene Creed.) This creed was attributed to the great St. Athanasius for a long time, but modern scholars (who I can't say Credo to) disagree.
The portion of the Athanasian Creed that concerns the Trinity reads: "The Father is God, The Son is God, The Holy Spirit is God; God is the Father, God is the Son, God is the Holy Spirit; The Father is not the Son, The Son is not the Father, The Father is not the Holy Spirit, The Holy Spirit is not the Father, The Son is not the Holy Spirit, The Holy Spirit is not the Son." These truths about the Trinity are illustrated in the image known as the Shield of the Trinity.
More colorful is the shield of the Trinity shown painted on the shield of a soldier going into battle.
The important thing to take away from this brief discussion is that the Creeds contain essential beliefs for a Catholic Christian. The Apostles', the Nicene, and the Athanasian Creeds are used in various liturgical settings in the Catholic Church, and each was created for the purpose of teaching what we need to believe in order to be Christians.
 The Creeds are discussed in much more detail at Creeds and linked articles you can find there about the liturgical uses of the creeds and about the individual creeds. Remember the online version is from 1932, and that things changed enormously after 1962 and the Vatican II. Also see Wikipedia article called Creed, where the posts include much more recent information.
 Found in a Wikipedia article titled Nicene Creed\
 I can't say "Credo" to most modern scholars, who seem to me to have an agenda of trying to make their careers as debunkers of the truths that we hold most dear. But I digress.