Catholicism is a denomination within Christianity. Catholics are Christians, but because there are other Christian denominations, all Christians are not Catholic. Catholics and other Christian denominations have several common beliefs in what they believe and differences as well.
All Christians, including Catholics, believe that there is one God in three parts, who exists as the Father, as the Son and as the Holy Spirit. All denominations also agree that Jesus is the Son of God in human form. All Christians also believe that God is displeased with humanity; however, God will save Christians because of Jesus' sacrifice.
One important distinction is what Catholics and Christians believe that people must do to be saved. Catholics believe that people must lead a good life in order to get into heaven. However, some other Christian denominations believe that faith in Jesus is enough to lead to salvation and that God does not judge the faithful based on what they have done or not done in their lives.
Another major distinction has to do with who has the power to speak God's word. Most Christian denominations believe that God speaks only through the Bible. However, Catholics believe that God speaks not only through the Bible but also through the Pope and the Catholic Church.