What exactly are academic librarians? My apologies for the title of this question, but I didn't know how to turn it. The fact is this is actually a set of many small questions.

I have completed my postgraduate studies in France where, unless I am mistaken, librarians that work in an academic library do more or less the same work as, say, librarians that work in a public library.

When I started to seek information about faculty jobs in Canada, I was struck by the fact that librarians seem to be pretty much considered as faculty.

Is that a fact? Is it the same in the United States? Is in the same in different types of institutions, i.e. more or less research/teaching-oriented?

I have looked at a few faculty collective agreements, and it seems like they usually include librarians. They seem to have pretty much the same working conditions, e.g. in terms of salary, sabbaticals and workload. The last agreement I read specified that librarians have a maximum of 12 hours of "schedules student contacts", in the same article that specifies that professors teach a maximum of 12 credits per year.

I would like to know what exactly is the job of an academic librarian, besides acting as "regular" reference librarians. I suppose they must keep up with the progress in their field and be able to assess the relevance of journal subscriptions and book purchases, but that probably doesn't account for the missing 23–28 hours a week.

Do librarians actually perform and publish research? If so, would that be mostly in their respective field, or in library and library science? Do they have tenure, and is the process similar to the one applied to professors?

Also, how does one become an academic librarian? Do you need an MLIS like a "regular" librarian? Do you need a PhD, and if so, must it be a PhD in library and information science, or in a field related to the position?
At many (but by no means all) universities in the US and Canada, professional librarians (usually with a terminal Master's of Library Science degree or perhaps a PhD in some academic discipline) are treated as members of the faculty, including eligibility for tenure, participation in shared governance, etc. 

In many other universities, librarians are staff members and not considered part of the faculty.  In a few places, there is a mixture with some librarians on the tenure track and other librarians who might have an MLS degree who are strictly staff members.  Most libraries also have lots of support staff and student employees who aren't professional librarians.  

In places where librarians are considered faculty, they are usually expected to be involved in some kind of scholarly (research) activity and in teaching students.  For example, a librarian who specializes in archival collections might produce a catalog of a special archive and publish it.  Librarians often teach courses in how to use the library to do research and might be involved in teaching courses in library science to future librarians.