What Is It?
CERN is the world's largest particle physics research facility. One of Europe's first collaborative ventures, it was founded in 1954 by a group of 12 countries that has now grown to 20 (see box). Situated on the Swiss-French border, it is famous for its 27-km underground tunnel--and the Large Hadron Collider (LHC), which, although running hugely overbudget, is due to be operational by 2005.
Whom Does It Employ?
CERN has about 2600 employees, of which only 3% are research physicists. However, another 37% of the staff is described as applied scientists and engineers. For these positions CERN typically seeks individuals with backgrounds in physics, computer science, electronics, mechanics, cryogenics, ultrahigh vacuum, materials science, radiation protection, cooling and ventilation, civil engineering, operation of accelerators and associated equipment, and conventional and superconducting magnets. According to the Web site, about 100 new staff members are recruited each year. Because all these vacancies are advertised on the CERN site, it's a good idea to keep an eye on it if you're of the physics and engineering persuasion and looking for a job.
The CERN community is a lot bigger than its payroll, however. CERN's staff are outnumbered more than two to one by "users" and "associates": scientists spending time working at CERN who are paid by universities and research institutes in their home countries.
Where Do I Start?
CERN runs a number of research programmes for both first-degree and postgraduate students. The Summer Student Programme  offers students with at least 3 years of undergraduate physics, engineering, or computing education the opportunity to spend 2 or 3 months at CERN, working with a project team and attending some specially designed lectures. Meanwhile, undergrads in technical subjects who need to get a practical training project of between 6 and 12 months under their belts as part of their degree can do so by joining CERN's Technical Student Programme  in Engineering, Computing and Applied Science.
If you're a theoretical or experimental physicist and want to undertake Ph.D. research at CERN, then the Web site explains that you should attach yourself to an experimental team from a university or research institute that visits CERN in the course of carrying out its research. However, other types of physicists, as well as engineers and computer scientists, can carry out between 1 and 2 years of their Ph.D. research at CERN through the Doctoral Student Programme  in Engineering and Applied Sciences. To qualify you must be registered for a Ph.D. at a university in a member state, and you must have your own financial support: The programme only pays a subsistence allowance for the periods spent at CERN.
Fellowships and Beyond
Experimental and theoretical physicists do get a look in when it comes to CERN fellowships  to work in a CERN research group, although to be eligible they must have a doctorate. Applied scientists, computer scientists, and engineers don't need a Ph.D., although having one won't disqualify them. Fellowships are initially for 1 year but are generally extended for a second. In addition to the CERN fellowship programme, you can work there as an European Commission Marie Curie Fellow .
Another way to obtain a temporary position at CERN is to become an Associate --effectively a visiting scientist. Under this scheme your salary continues to be paid by your home institution, although CERN pays an allowance to assist with living away from home. Usually associates work at CERN for no more than a year; the absolute maximum duration of such an appointment is 2 years.
Although CERN has tackled its legendary budgetary problems in part by cutting back on the number of its own employees, the LHC project, which started in 1991, is employing 31% more applied scientists and engineers now than it was then. Vacancies  for all open-ended appointments at CERN are conveniently divided by the required background of recruits they're looking for: physicist, university engineer, etc.
Anything Else I Should Know?
CERN's official languages are English and French, so if you want to work there you'll need a good working knowledge of one, and it's desirable that you have a basic knowledge of the other (or demonstrate a willingness to acquire it ... CERN runs English and French language lessons for its employees).
And Yet More Training Opportunities
CERN is involved in a number of regular workshops aimed at young scientists, several of which rotate around Europe or even the world. The European Schools of High Energy Physics  are held in a different European country every year. The CERN Accelerator School  holds courses twice a year and every 2 years is one of the organisers of the U.S.-CERN-Japan-Russia Accelerator School. The CERN School of Computing  is held annually in September.
Any Other Interesting Stuff?
Science in the Big League , an essay written by Norwegian particle physicist Egil Lillestol, who has spent a lot of time at CERN, is a great introduction to CERN's multinational environment and the joys and challenges of doing science on such a big scale. (NB. The little right-facing arrows at the top and bottom of each of the essay's pages take you to the next one; this wasn't immediately obvious to me.)