Invented by Johann Gutenberg, the Gutenberg press was a huge leap forward in printing technology. It wasn't the use of the press itself that was revolutionary, but rather the practical use of moveable type. With moveable type, symbols and the letters of the alphabet are carved into individual blocks. These blocks can then be arranged to form words and lines. Before this, each page had to be carved into a unique wooden plate.
Movable type did exist before the invention of the Gutenberg press. Gutenberg's great innovation was the development of a method to make all the letter blocks a uniform depth, ensuring that they all made good contact with the page.
The first books printed by Gutenberg were copies of the bible. As the press was quick and simple to use, the cost was low enough that peasants could afford books for the first time. This lead to an increase in critical thinking, and removed the need for all religion to be interpreted by priests.