I am visiting an American university for a year. In my invitation letter and further instructions, it is written that I can audit any course that I want (subject to the agreement of its professor).
Since it is not common in my country, I have no idea what this auditing process is. What should I do?
Is it an official process with official report, or do I simply attend the class like students?
It usually means that you can sit in the lectures, but that none of your work will be graded/marked and you won't get any credit for it. Some universities have official forms that record the fact that you audited a course. Others just do it through personal consultation with the instructor.
During my first year of grad school, I audited a codes and cryptography course my senior-year undergrad friends were taking. I sat through the crypto part and then bailed on the codes. It was nice. I know a little about RSA and Elliptic Curves now. I got to hangout with my friends and I didn't have to do any work. I think I'm a better educated person for it, but it doesn't really affect my career.
Auditing a course means a student can take classes but cannot be graded or given credit for a particular course. It is usually done for academic exploration and self-enrichment.
From Wikipedia:
In academia, an audit is an educational term for the completion of a course of study for which no assessment of the performance of the student is made nor grade awarded.
Audit has several meanings.
1a. Formal audit: costs money, but only a fraction of the normal price. On the other hand, there is no financial aid possible. You are expected to do all the work and take quizzes and tests, and you may participate in class, but there are no repercussions if you stop going or don't hand something in or take an exam. When you audit a class, there's no grade on a transcript, and no academic credit is earned.
1b. Informal audit: free. You phone, visit or email the instructor and ask if it would be okay to do an informal audit. Many instructors will accept this. Things are similar to 1a. Key difference: you wouldn't get a library card or a university computer userid.
Those are the most common ways one might talk about auditing a course. But it's possible they meant something else. If you're going to attend as part of an exchange program, it's possible they meant that you can choose whatever courses you are interested in, and have the requisite background knowledge for, without having to pay the U.S. university directly for.
I don't want you to be alarmed and assume that you will not get academic credit for the courses in the U.S. university. That might well not be what they meant. You should definitely ask them what they meant.
As I understood from several discussions here on Academia.SE it means that you can attend whatever classes you want but you are not allowed to take the exam, see e.g. this answer.
It should be noted that in several countries auditing is granted by law in public universities.
The meaning of "audit" is very unclear, even if we restrict attention to the U.S. The other answers do attest to certain realities in certain situations, and to the variation. Perhaps to give an overview:
"Audit" at a minimum just means "hearing" the lectures. It is completely unclear whether or not you have to pay, whether or not you have to do homework, quizzes, or exams, whether or not you are allowed to do those tasks and get feedback, whether or not your official transcript will show that you "audited" the course(s), and so on.
The particular question of whether you "have to do any work" is funny. Indeed, in most audit situations, an auditor is not commanded to do anything at all. That is different from the question of whether a sensible person could see the point in engaging with the material. Truly, the model of "listening" which is almost completely passive has the virtue of very low overhead, and may be appropriate in some situation. But, if one wants more, and has the time and energy, the fact that one is not required (by external authorities) to do something does not mean that one cannot choose to do it.
For that matter, many universities' official rules (often, in the U.S., that people have to pay something to be "allowed" to attend a course, even if they make no further demands on the instructor) are not uniformly conformed-to by faculty. That is, the official description of the rules-of-the-game may not be correct at all "on the ground".
The one most likely common feature is that "auditing" a course will not provide you with any certification that you are competent or expert in the material. (We can wonder whether "good grades" ever did certify this...) Thus, if one views "education" as a process of obtaining certification, auditing is not directly purposeful. On the other hand, if one views "education" as a process of acquiring information, auditing is nearly as good as any approach, except for the possible lack of feedback from the instructor. (In fact, in many situations, the "exercises" and such are significantly make-work, exactly because the instructor is tasked with generating a steady stream of "work", so it's not clear that anyone should be terribly interested in "feedback" apart from anticipation of the eventual "grade" and/or success or failure of certification.)
