Post production is everything that happens to a film once shooting ends and the cast and crew go home. It involves the editors who work with sophisticated computer technology to put together hundreds of hours of raw footage into a cohesive and exciting story, it is the musicians who compose the score, it the sound effects editors and folly artists who add image-enhancing sounds when the original noise was not sufficient and it is countless other assistants and project managers who make up the latter half of a film's credits.
These technical positions are often unionized and require several years of apprenticeship before accreditation. Upon being formally inducted into a union, a person is eligible for union wages. Procuring jobs is often based on who one knows -- directors often select teams for their films who they worked with in the past. Those relationships end up being extremely valuable since people often work with the same individuals again and again.
Many post-production artists are film school graduates that either specialized in coursework related to one particular area, or are people who were brought into apprenticeship programs through friends.
The 'crew' of any production is the behind-the-scenes individuals who work on the set of a movie, film, TV show, or video production. This is separate from the 'cast,' the actors or performers in front of the camera. There are often intriguing titles in a film's credits (e.g. boom operator, best boy, grips, etc.). They also include the usual expected characters-the costume designers, the makeup artists, the hairdressers and so on.
The most important members of a crew (aside from the director of course) is the cinematographer, a.k.a. the director of photography (more commonly known as the D.P.). This is the individual that creates the visual texture of the film and makes each scene aesthetically appealing. Boom operators, grips, and best boys are often the 'muscles' behind a production -- generally men who work with the technical details of sound, projection, lighting and other aspects of production.
There are a few other key roles -- one is the Assistant Director and the Unit Production Manager. The A.D. works with the director, and often has the opportunity to direct secondary footage (establishing scenes, the introductory credits, transition scenes, etc.) The Unit Production Manager manages the logistics of making everything happen for the director -- ensuring that the location is suited for shooting, that all permits have been met, that everyone working on the set is doing so within union guidelines, managing payroll and the shooting schedule, ensuring that catering and ancillary benefits for the cast and crew are accommodated, and making sure that all the requisite production necessities are on hand (i.e. trailers for stars, headphones and walkie-talkies, etc.)
The lowest level of entry onto the set of a production is as a production assistant. Any given production will have about a half-dozen PAs, some paid, others not. Many of them will be friends of crew members who simply want credits on their entertainment resumes. PAs are the production parallel of creative assistants. They do everything from deliver scripts to relevant parties, fetch coffee, and do any other menial tasks that need to be done.
As in post-production positions, most crew members are unionized. For those fortunate enough to be in the loop, it can be a glamorous job that enables a person to travel to exotic locations for film shoots, enjoy several months of vacation a year in between projects, and to earn a healthy salary in the process. The downside, of course, is that projects are uncertain and the hours are very long while working (film shoots are known for being pressured sessions of 14 hour days for 30 days straight). There are some unique programs to enter the field, such as the Assistant Director Training Program which is sponsored by the Director's Guild of America. About two dozen individuals with some experience in the entertainment industry and with goals of eventually being Unit Production Managers (the title of the program is a bit of a misnomer-the application clearly states that it is NOT for aspiring directors), are given the opportunity to work for a year on major productions.