SAG-AFTRA Is the Union representing all actors employed on UNION recognized work in the US. You must be a fully paid member of SAG-AFTRA to work on these jobs. The acronym stands for Screen Actors Guild and the American Federation of Television and Radio Artists.
Essentially to join you must first work a SAG-AFTRA job, when you are offered your first union job, you are urged to consider joining SAG-AFTRA. This is a commitment though, once you are a member, "you must abide by the rules of membership, starting with Global Rule One. And, whether you are a SAG-AFTRA member or not--never accept work during a union strike!"
After this first job there will be initiation fees and they will check the legitimacy of the Union job you did. There are huge benefits to being in the Union such as Scale Rates (set rates per job), health insurance (if you earn enough for the Union), monitored residuals (e.g. you get paid every time the spot airs) and general protection from low rates and delayed payments etc. The downsides are that if you are solely SAG-AFTRA, you can ONLY work on SAG-AFTRA jobs, and that the fees you pay can be quite hefty.
The main Union/non-Union differences are that an actor who is non-union may NOT work on a Union job. Conversely, a Union member MAY work on a non-union job if they are FI-CORE (See 'What's FI-CORE?').