Here's a quick overview:
Catholicism: The original Western Church, headquartered in Rome, traces its origins back to the ministry of St. Peter and Paul there. Believes in the apostolic succession, which essentially means that the clergy are seen as having the spiritual authority of the original disciples. Highly hierarchical with rigidly established doctrines.
Protestantism: Broke off from the Catholic Church in the 16th century and typically differs on a few core issues:
- does not recognise the authority of the Pope (some Protestant denominations are vehemently anti-Pope, while others respect him but don't acknowledge him as vicar of Christ)
- does not believe in the use of images in worship (i.e. icons and statues in church)
- does not believe in praying to saints or venerating Mary
- allows priests to marry, and (except for in the case of High Church Anglicanism, which can't be considered truly Protestant anyway) doesn't believe in monasticism
- tends to have a less ornate liturgy
- has a much stronger focus on scripture as a source of authority
- does not believe in unscriptural Church tradition as a source of doctrine
- believes in salvation by faith alone
- does not believe that priests can intervene on God's behalf to forgive sins, and so does not believe in the sacrament of confession
The cornerstones of Protestantism are summed up by the five solae propounded in the Reformation: sola scriptura, sola fide, sola gratia, solus Christus and soli Deo gloria. There are however so many different branches of Protestantism that you can find some that disagree with basically any aspect of it. Once they had got into the habit of splitting off, there were no limits