A journalism school is, unsurprisingly, an academic institution that teaches journalism. Journalism school can take a variety of forms, from community colleges to universities to graduate schools. Depending on how they're organized and what kind of accreditation they have, journalism schools may offer certificates, diplomas or degrees.
The curricula across journalism schools may vary widely, but most probably teach how to gather and report news for different media (print, broadcast, online, et cetera). Major journalism schools tend to contain some kind of practical component, possibly giving students real-world assignments that would ideally be suitable for publication somewhere, and/or including an internship or other real-world experience as a graduation requirement.
Journalism programs in a university or college are likely to include some form of liberal arts requirements, helping to give students a broader knowledge base that they can draw on in their journalism career. (The benefits of this vs. real-world experience are up for debate.)
Most journalism schools will have mandatory courses in media ethics and law. Other offerings will differ from one institution to the next; my other core journalism courses included newspaper reporting, radio broadcasting, TV broadcasting, online journalism, feature writing, copy editing and newspaper layout. I majored in newspaper journalism and also had to complete a six-week internship at a major newspaper and work on a six-week masthead project producing the school-run newspaper. Because this was happening at a university, I also took English, history, sociology and philosophy classes. The music history class was my favourite.
A good journalism school will employ instructors who have been active in the industry and can speak to its practices, provide relevant advice to students and potentially assist with networking. Most schools with a journalism program have at least one student newspaper, either run by the school or independently.
A journalism diploma or degree is often a listed requirement in journalism job postings. As to whether a journalism school will seriously improve one's prospects of a successful journalism career, read SharQ's extremely thorough writeup on the subject.
dannye used to have a writeup here. His lead sentence was "What killed journalism." I first came across it while I was in journalism school, and it was somewhat disquieting — though he made some interesting points, specifically that journalism schools became meeting grounds for idealistic activist types who were less interested in being objective reporters than in changing the world.
In my own experience, I learned a lot and use much of the knowledge I acquired in classes in my career. I also found other classes to be less useful, and feel that my time at the independent campus paper (not run by the school) was as educational if not moreso.
My main contention would be that journalism schools teach a highly specialized skill set, and pursuing it as an area of undergraduate study can leave someone without other areas to fall back on should they decide they want to change careers after spending some time in the industry, making other training necessary. A journalism education is transferable to other careers, but the options are limited.