What exactly he invented
Printing from wooden blocks was already known in China and Korea. Gutenberg's contribution was the system as a whole: a hand mould for quickly casting identical metal letters, an alloy that set fast, oil-based ink and a press adapted from a wine press. The letters could be set into lines and used again.
Consequences
Before printing, copying a book by hand took months. Once printing arrived, print shops spread across Europe within a few decades. Scientific knowledge, the Reformation and newspapers spread quickly. The written word became a mass product.
The first printing in Slovakia
The first print shops on the territory of Slovakia appeared in the 16th century. The printing press is the first step on the long road to today's internet.