Protestantism started with the Reformation of Martin Luther. Protestants accept the authority of the Bible, but reject the authority of the Pope. Without a single unifying force Protestants have split into countless different sects over the centuries. Liberal protestants are Liberal Christians and have no strong authority figure. Neither the Pope nor the Bible control them.
Protestant fundamentalists by contrast see the Bible as literally true and inerrant. The leaders of their particular Protestant sect decide for them how to interpret the Bible. Many of the more extreme fundamentalist Protestant sects are considered cultlike and harmful.
There are roughly 500 million Protestants worldwide and 53% of Americans are Protestant.