Bitcoin Q&A: What is a Fork?

What is a fork? What is the difference between a client fork and a network fork? How do hard forks create new coins? What are examples of consensus forks? Does Bitcoin ever go down for maintenance? How does the community come to consensus, particularly regarding soft forks?
These questions are from the MOOC 9.5 session which took place on March 16th, 2018. Andreas is a teaching fellow with the University of Nicosia. The first course in their Master of Science in Digital Currency degree, DFIN-511: Introduction to Digital Currencies, is offered for free as an open enrollment MOOC course to anyone interested in learning about the fundamental principles.