Catholics are Christians. That's like saying, what is the difference between Pitbulls and dogs. Pitbulls are a type of dog, as Catholics are a type of Christian. Catholics, Baptists, Methodists, Lutherans, and Presbyterians all believe basically, that Jesus died for their sins, which makes them all Christian "denominations". In other words, they all basically believe Jesus was divinely born, lived, and died for their sins, but have their own slightly different spin on either the story, or the responsibility of a faithful Jesus worshiper. Thus they split off into different denominational groups to worship how they see fit.
For example, in the sixteenth century, a German Christian monk named Martin Luther was so disgusted by a blatantly corrupt and for-profit Roman Catholic Church, he started his own denomination to be more faithful to Jesus' intentions with the faith. They are now called Lutherans.