Basically it comes down to authority.
Catholics believe in the Scriptural supported, traditionally upheld teaching that the Church was instituted by Jesus as His Authority on earth, to be led by His ‘vicar’, the Pope.
Non-Catholics believe in some other authority. Some believe in a ‘group’ of bishops, some believe in the Bible, some in a creed. . .but they do not believe in the authority of the Catholic Church.
Some non-Catholics believe in a lot of the same faith elements we do; some even use the Catholic Nicene Creed in their service which speaks of the One Holy atholic and apostolic church, but they believe in a so-called ‘universal’ church which could include Catholics, but also include others equally and there is some kind of ‘invisible’ universal church, not an ‘earthly’ one like the Catholic Church.
Some nonCatholics do not believe in many of the elements we have, and instead believe in many other different ones based on their personal interpretations of Scripture.
Depending on the individual Protestant (it used to be said there were 36,000 different types and while that is an exaggeration based on counting some groups which are part of ‘larger’ groups, the fact is that there are a great many different churches, even if some of them only differ by a few elements; often the differences can be striking with one group of the two differing on whether baptism is for infants or adults, say), that individual’s difference between the Catholic faith teachings can be small or huge.
And of course, many who identify themselves as Catholic do not practice all of the faith teachings, so they can look even more like a mainline Protestant. The difference between an Episcopalian whose church accepts abortion, divorce and remarriage without a decree of nullity, gay marriage, and women priests as doctrine, and a baptized ‘practicing’ Catholic who ALSO accepts abortion, divorce and remarriage without a decree of nullity, gay marriage, and women priests as something to be desired, even though it totally contradicts the faith he says he supports, can be almost non-existent, though the teachings of the Episcopal Church and the Catholic Church on those subjects and others is poles apart.