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 practicle use of moveable type. With moveable type the letters of the alphabet, and symbols, are carved into individual blocks which can be put together to form words and lines. Before this each page had to be carved into a unique wooden plate.
Until the invention of the Gutenberg press movabletype was possible but Gutenberg developed a method for making all the letter blocks a uniform depth thus ensureing they all made good contact with the page.
The first books printed by Gutenberg were copies of the bible and as the press was quick and simple to use the cost was low enough that for the first time peasants could afford books. This lead to an increase in critical thinking and removed the need for all religion to be interpreted by priests.