Us Catholics have a Pope; the protestants don’t. I’m not sure if the protestant religions even consider us proper Christians (edit: Of course we all believe in Jesus; what I meant by the last sentence was that I’ve been to places where if you say “Christian church”, it refers to a place of worship that is protestant, but not Catholic).
Catholics were around first, until the 1500s when some guy named Martin Luther started a movement that created protestantism. The protestant movement started because some people didn’t like the way the Catholic Church handled things and I guess they wanted to get more back to basics (that is, focus more on the Bible rather than all the Catholic traditions) - that last part may be my personal opinion.
The protestants have a common set of 3 fundamental beliefs: that scripture (the Bible) alone is the source of all authority (unlike Catholics that have a Pope and a Church that can decide some stuff), that faith in and of itself is enough for salvation, and the universal priesthood of believers (which means that any Christian can read and interpret and spread the word of God, unlike Catholics which have a dedicated priesthood).
Among protestants they have different denominations - Baptists, Presbyterians, etc. They all observe the same fundamental beliefs mentioned above, but they vary in their practices and on what stuff they focus on.