The development of the printing press is, in my opinion, the event that has had the biggest and most lasting impact on today's society. We say that knowledge is power; without the printed word, it is difficult for any society to obtain and to maintain widespread knowledge (although not impossible). The printed word is a tangible, concrete and lasting way to acquire knowledge and to transmit it.
Of course, books existed before the invention of the printing press. However, those were handwritten and richly decorated through a long drawn out, painstaking process, using expensive materials. Because each book was created by hand, each was really a unique work of art. They were created by monks, and were strictly reserved for the very rich or for those religious orders that created or collected them. In other words, they were very rare and used by a select few. In this way, knowledge was restricted to those who could afford it. Of those, few could even read them.
With the printing press, books could be mass produced, using cheaper and more readily available materials. They could be sold to anyone who would pay for them. Through mass production, they could reach more people, thus spreading knowledge to those who would have been excluded from it before. Moreover, since anyone with the skills and equipment could print books, subjects became increasingly varied, and new ideas began to spread, such as philosophy, sciences, and others. As books became more available, literacy increased and an interest in learning followed.
The invention of the printing press was truly revolutionary. We owe much of what we know today to this momentous event.