Extras aren't always professional actors. In fact, most extras are just regular folk who wanted to be a part of the film and television industry. Extras are paid a fairly small wage for their participation and they are usually expected to stay on set from the beginning of production until wrap (the end of production).
Depending on the type, size and budget of the production, the extras may or may not take part in make up, wardrobe, hair, etc. Often many period pieces will entail that the extras are "fully dressed and fitted" which means that they are provided with costuming by the wardrobe department. But more often than not, extras are simply informed beforehand of the type of clothing they will need to bring and asked to furnish it themselves.
So, none of this seems all that glamorous, right? Well, the fact is that being an extra really isn't all that glamorous. In fact it's probably one of the least glamorous positions on a set. That said, why would anyone in their right mind want to do such a job?
Probably the best reason to take a job working as an extra is it gives you the ability to network with dozens of fellow people who are all trying to break into the industry in one capacity or another. The contacts you make here might lead to a number of other opportunities down the line as many of the people you come across could perhaps find success in their niche and thus be able to help you in your own career endeavors.
If you're new to the industry, no matter how many books you read or classes you take, you can never fully understand the inner workings of a film or television set unless you spend a great deal of time on one. There is a reason Hollywood is called a "factory." The main reason is because that much of the work that is done on a set is manual labor. From lugging cable and camera dollies to adjusting lights, props, etc., a movie set is a factory churning out film and television as its product. Being an extra on a set will expose you to this reality, as well as let you see first hand what these people do. You might find that there are particular jobs you never even thought of that are appealing to you.
It's not likely, but it has certainly been known to happen that someone may spot you and realize that you are meant for much more than just background. From casting directors to writers to agents wandering the set, you never know who has their eyes on you. So, being on a set in this capacity might just open up a few doors you never thought possible.
How to Get The Job:
Extras are hired either as individuals for a particular scene or in groups for a series of shots. There are "Extras Wrangling" companies which are those companies that specialize in finding extras of a particular age, appearance or ethnic background. By registering with these companies, they simply contact you when an opportunity with criteria that matches your particular physical description comes up. You are then given a call sheet by the production office and told when to report to work.
On most sets, your "boss" will either be the Second Assistant Director or Extras Captain (or Wrangler -- or it could even be under a different name -- it depends on the production company). Make things easy on yourself and on them by doing exactly as you're told. They will be very specific about things like when to come out, where to go and whom you should and should not speak to on a set. This directive will be made very clear to you by the way as it's distinctly possible you could be made an extra on a production that has a major star roaming around and the last thing they want you doing is approaching them uninvited. More often than not, you'll simply be replaced if you ignore this directive.
That said, it's an easy, yet tiring and thankless job. However, if you're simply looking for any way to be a part of a Hollywood movie or television set than becoming an extra might just be your ticket in.