There are many differences between Roman Catholicism and Protestantism, and the history is quite complex, but if I had to simply things, I would say that there are two major differences.
1. Catholics believe in a visible universal church. In other words, all Christians around the world fall under the same church structure. There is one pope who has authority over all Christians, and under the pope there is a complicated hierarchy consisting of archbishops, bishops, cardinals, priests, etc. On the other hand, Protestants believe in an invisible universal church. In other words, there is no one unifying church structure for all Christians. The church is universal in the sense that there are Christians all around the world, but it is invisible because there is no structural link between them. Therefore, Protestants do not recognize the pope. Instead, they choose to be independent local churches, although many of these churches also choose to link themselves to other similarly-minded churches to create denominations.
2. This is perhaps the more fundamental issue. Protestants base what they believe on the Bible, while Roman Catholics base what they believe on both the Bible and the church tradition. In addition, while the Protestant Bible does not include the Apocrypha (a series of books between the Old and New Testaments), the Catholic Bible does. What I mean by "church tradition" is that much of what Catholics believe is not actually found in the Bible, but it has been the gradual development of doctrines set forth by Catholic leaders and theologians throughout their 2000-year history. Most notably, Catholics would hold councils that would in a sense introduce new doctrines to the church. On the other hand, Protestants do not believe that any creeds, councils, popes are binding for people today. They only accept the Bible as the Word of God, and all issues not mentioned in the Bible are left to human wisdom and judgment. Of course, some Protestants do use creeds or councils as guides, but such creeds or councils are never seen as introducing new doctrines but as presenting doctrines from the Bible.
For example, there is nothing in the Bible concerning what language people are to use in worship services. Therefore, when the Protestant Reformation started in 1517, one of the major issues for the early reformers was that they believed church services should be conducted in people's vernacular languages. To the Protestants, the lack of mention of worship service languages in the Bible meant that they had the freedom to use whichever language suited them best. On the other hand, the Catholic masses had been conducted in Latin, even though the majority of people did not read or write Latin, because this was the established church tradition. The Protestants did not see church tradition as binding. However, in 1962, the Catholics held the Second Vatican Council, and one of the things that was decided was that masses were to be conducted in people's vernacular languages. From then onward, Catholics have stopped using Latin in their masses, for the most part.
In other words, over the past 2000 years, the