I grew up Catholic and now go to a Protestant or non-Catholic church in Charlotte, NC. Michael Houdmann, Founder and CEO of GotQuestions.Org writes that a major difference between the two boils down to believing in the Bible alone as God's word verses believing that God's word plus man's input as the divine inspired word of God.
WHAT DO PROTESTANTS BELIEVE?
A major difference is the issue of sufficiency and authority of Scripture. While Protestants believe that the Bible alone is the source of God's special revelation to mankind, it also speaks to everything we need to understand as necessary for our salvation from sin.
Protestants view the Bible as the STANDARD for Christian morality and ethics which governs our behavior. This is called sola scripture and is one of the five Solas in Christianity. (Solas meaning "alone" in Latin. 2 Timothy 3:16-17 makes clear that "All Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness, that the man of God may be complete, equipped for every good work” (ESV).
WHAT DO CATHOLICS BELIEVE?
Catholics do not believe in sola scriptura hence do not believe that the Bible alone is sufficient. They see both the Bible and sacred Roman Catholic tradition as equally authoritative and binding on the Christian. While not all Catholics hold to doctrines such as purgatory, praying to the saints, worship or veneration of Mary, etc., the Catholic doctrine hold to them even though they have little or no mention or basis in the Bible. In truth, these doctrines are based on Roman Catholic traditions based on the Catholic Church's insistence that the Bible and tradition are equal in authority. The problem with this is that it undermines the sufficiency, authority, and completeness of the Bible. The view of Scripture is at the root of many, if not all, of the differences between Catholics and Protestants.