As people have said, Catholicism is just a specific type of Christianity. As for the cross bit, many Protestants use crosses as opposed to crucifixes (the cross with Jesus on it) because of the commandment that says you shouldn't make any graven images. Catholics interpret this to mean you shouldn't worship any graven images and use the crucifix to help focus their thoughts during prayer or worship. Some other points based on the differences between Catholics and Protestants (most Christians who aren't Catholic are Protestant):
1. Saints. Catholics will pray to saints, asking them to intercede on their behalf. Protestants don't do this, and don't really have saints. I believe one reason is they sometimes think of this as worship, naturally forbidden. Also, Catholics believe Mary was born without sin (the "immaculate conception"). Protestants do not.
2. Structure. Catholicism has a hierarchical structure with the pope at the top, then cardinals, bishops, priests, deacons, other religious, and laypeople. This is not to say Catholics follow the pope mindlessly or above God, just that he acts as a father and a guide for the Church. Protestants do not have this structure, but rely on small self-governing communities.
3. The Eucharist. Catholics say that the Eucharist is the essence of Christ, present with them. They worship Christ in and through the Eucharist. Protestants (generally) think the Eucharist is bread, symbolic of Christ, but not actually Christ.
4. Salvation. Generally, Protestants believe that salvation can only come from accepting Christ as your savior. Essentially, no matter how good a person is on their own, because they're not perfect, they wouldn't be able to make it to heaven if it weren't for Chirst's sacrifice. Catholics believe this is true, but that you also have to 'walk the walk, not just talk the talk.' Salvation is based on the acceptance of Christ as savior combined with good works.