The break within the Christian Church began with Martin Luther posting the 95 Theses (or complaints) against the Catholic Church. At the time, the Catholic Church was very corrupt, for example, the selling of indulgences.
Protestants do not pray to saints, but instead directly to God. There is no pope or "head" of Protestant denominations. There are also differences in doctrine and practice.
One of the main differences are in doctrine. Protestants believe in being saved by faith, Catholicism is a works based religion. You have be baptized, first communion, confirmation, ect.. To be saved. Also, Protestants believe that the partaking of the bread and wine as Jesus body and blood are symbolic where as Catholics believe the bread and wine actually turn into the flesh and blood of Jesus.