From the Introduction:

In the early days of artificial intelligence, the field rapidly tackled and solved problems that are intellectually diffcult for human beings but relatively straight-forward for computers—problems that can be described by a list of formal, mathematical rules. The true challenge to artificial intelligence proved to be solving the tasks that are easy for people to perform but hard for people to describe formally—problems that we solve intuitively, that feel automatic, like recognizing spoken words or faces in images. 

This book is about a solution to these more intuitive problems. This solution isto allow computers to learn from experience and understand the world in terms of ahierarchy of concepts, with each concept defined in terms of its relation to simpler concepts. By gathering knowledge from experience, this approach avoids the need for human operators to formally specify all of the knowledge that the computer needs. The hierarchy of concepts allows the computer to learn complicated concepts by building them out of simpler ones. If we draw a graph showing how these concepts are built on top of each other, the graph is deep, with many layers. Forthis reason, we call this approach to AI deep learning.