The Catholic concept of God is, as the Baltimore Catechism sums it up, ‘a spirit infinitely perfect’. By this is understood that God is uncreated, extant outside time and through it, omniscient, omnipotent, and omnibenevolent, possessed of all good qualities in infinite degree with none of the bad. Further, God is necessary: without his interest and specific action, we wouldn’t be here talking about this.
More specifically, Catholics believe in a triune God, one who is three – the persons of the Father, the Son, and the Holy Spirit subsisting in one divine nature. The person of the Father is the Creator; that of the Son, the Redeemer and Logos, the Word of creation; that of the Spirit, the manifestation of the transcendent love between Father and Son, and sadly the one who gets short billing in the Apostles’ Creed and whose origin is the subject of much contention with your siblings in faith, the Orthodox Churches. You believe the Spirit proceeds from the Father and the Son; they drop the filioque clause, and the world has not heard the end of it yet.
I suppose growing up Catholic and having had to memorize large chunks of the Baltimore Catechism is a form of cheating, but there you have it.
As to why you believe: that’s a tougher question, and certainly not one a non-believer is equipped to answer. The Catechism (the non-Baltimore version) says The desire for God is written in the human heart, because man is created by God and for God; and God never ceases to draw man to himself. Only in God will he find the truth and happiness he never stops searching for; but frankly, if that were true there wouldn’t be an atheist or agnostic in the world. So, why do you believe?