The Screen Actors Guild is a union. It's a union of actors, producers and crew. The union itself does not make movies. SAG enforces minimum wages, collects and distributes residuals, enforces standard working conditions and offers health care.
In Los Angeles and New York, most productions are "closed shops". That means you must be a member of the SAG union to participate as an actor, or that minium numbers of SAG members must be hired before non-union actors are hired. You need to have a credit as principal from a SAG production to become a member. So, it's a bit difficult to join when you need SAG credits, but cannot work unless you are a SAG member. Often it's easier to to get SAG credits in a right to work state like Georgia, where SAG cannot operate as a "closed shop". You can get credits on SAG projects, then become a member and pursue roles in New York or Los Angeles.
Conversely, SAG members are not to participate in non-union projects. The Screen Actors Guild refers to this a rule one. That provides the encouragement for most major productions to become SAG productions. Any production that is non-union has eliminated a large segment of the working actors from casting.
In addition to the credit requirement to join, there are union dues. A one time initiation fee is charged to join. Yearly dues must then be paid to remain a member of the Screen Actors Guild.
A similiar union is the American Federation of Television and Radio Artists or AFTRA. While SAG covers the film industry, AFTRA covers television and radio. AFTRA provides the same protections for those working in the mentioned mediums.
