I assume you're asking about the Catholics and Protestants, rather than Christians and Protestants, since Protestants are Christians, too. I also assume you are not asking about a specific literary work, since you don't name any. You must be asking about the issues involved in the Prostestant Reformation, then.
This topic is extremely complex, and depends partially on which side's point of view you have. But I can give you a few essential issues:
- The authority and role of the Pope. Catholics see the Pope and other figures of the hierarchy as church leaders. Protestants do not.
- The authority and role of church tradition. Catholics place a more important role on tradition than Protestants do. Protestants tend to use the Bible as the sole source of authority.
- Transubstantiation. Catholics believe the communion wafer transforms into the body of Christ, while Protestants believe the wafer simply symbolizes the body of Christ.
- At the time of the Reformation, indulgences were a major issue. Defining indulgences really depends on your point of view, however, and I'll leave that for you to research or someone else to define.
Those are a few of the essentials. They are just the beginning, however.