The question was originally posted in Quora and I could not resist writing a response (seen below).
I believe the perspective of a professor’s job being relaxed comes from the fact that most people only interact with their professors during lectures, i.e. at most 3 hours per week. So, it would seem like the professor just has to show up to class for the lecture, talk about something (which they repeat every year since they are teaching the same course again), and then do nothing else. Thus, professors should just be able to relax for the rest of the week (i.e. 37 hours are free assuming a 40-hour work week—which is typically an underestimate). When I was an undergraduate student, I had a similar perspective until I started working with a professor as a research assistant.
In reality, lectures make up a very tiny fraction of the job of a professor. Even within teaching, every hour of lecture takes anywhere from two to eight hours of preparation. Besides teaching, professors have numerous other responsibilities which can be categorized as either research or service.
Many of the professors are in the job because they enjoy research. It gives us the opportunity to work on something that nobody in the world has done before. It allows us to be creative and inspire creativity in other students. Research is how professors are judged in the academic world (i.e. around their professional peers). The impact is measured by the peer-reviewed publications (journal articles), conference proceedings, and invited talks. To conduct high-impact research, professors typically run a research group (and usually a lab as well), which requires funds. These funds come from grants provided by the federal government or support from industries. Raising funds requires writing grant applications (which takes up a significant chunk of our time). Each grant proposal can take 50–100 hours to prepare and usually we are writing about 10–20 proposals a year. If we don’t raise these funds, our research group members (students) will not get paid nor will we be able to purchase supplies or equipment for the lab. (See what happens from grant writing to research publication in this article by my colleague)
On the service side of things, our responsibilities include serving on at least half a dozen different committees. Each of them has a meeting every couple weeks or so and we have tasks assigned before attending these meetings. For example, we will serve on admissions committees, review applicants, and discuss them in these meetings.
The list of tasks goes on and on, but the biggest thing is that we do have our independence and that freedom is the most self-motivating driver. I actually started this blog specifically to share some of these experiences of our jobs.