Eric told you about the school library route, and Cthulu's Librarian told you about the academic route. There are two other major categories, public libraries and special libraries.
I'm a public librarian, so I can tell you most about that. I currently manage a branch library in a county library system. Like Eric, I have my own little kingdom. I do everything for my branch from hiring (and firing) to ordering the books and other materials that go on the shelf. Essentially, the buck stops with me. It's my job to see that the employees are well trained, that we are providing the right kinds of services, such as story times and book discussion groups, for our neighborhood, and that the appropriate materials are in the collection. I presently have a full time YA librarian working for me (YA=teen in library jargon) and two full time children's librarians. I also have a part time paraprofessional (bachelor's degree) and 2.75 FTEs of clerical staff. For me, a typical week involves spending time on the reference desk answering questions, attending committee meetings, fiddling with scheduling and bugetary stuff, collection development, and even some time at the circulation desk. Of course, since I'm "the boss" I also delegate tasks to other employees, and handle personnel issues from cheerleading to reprimanding (rare).
I have a bachelor's degree in Medieval English Literature and an MLS from the University of Kentucky, oddly enough.
I don't know if EN World counts a special librarian among our members, but here's what I know about it: This is a widely varied category, which includes medical librarians (they work in hospitals serving the doctors, nurses and students, not the patients), law librarians, who actually aren't usually lawyers, though sometimes they are, corporate librarians (every big corporation, from Kraft to Eli Lily has at least one and usually far more librarians on staff). While corporate and law librarians make far more money than school, academic, public or medical librarians, they don't have much job security. So it comes down to a personal choice. Make 80-120k a year but lose your job as soon as the economy starts to get shakey, or make in the 40-70k range but be insulated from that sort of trauma. And then too, if you're an academic librarian, you likely have the whole 'publish or perish' pressure since you are probably considered faculty. Blech.
That's it in a nutshell. Ask if you have more questions!
edit: spelling
