In several credits, some names (usually among the first ones) have an abbreviation after their name like:
Where does it come from?
Do they mean something specific about their job?
Generally, these abbreviations you see indicate a society or union that person is a member of. Since such societies are very distinctive and respected ones, people add it to their names in credits. Just like when one has Ph.D. degree, he/she would prefix Dr. or suffix Ph.D. in his/her name.
It stands for American Cinema Editors, a society for film editors. This is a high honor in the cinema industry.
It stands for Association Française des directeurs de la photographie Cinématographique (in English, French Society of Cinematographers). It is a France's foremost professional organization of French cinematographers and founded in 1990.
It is the American Society of Cinematographers for special effects experts and photography directors. This organization was established to build a collaborative environment for cinematographers to share their ideas and techniques.
The organization is not a labor union or guild, but an educational, cultural, and professional organization. Membership is extended by invitation, exclusive to those who have demonstrated outstanding ability as a director of photography.
British Society of Cinematographers. Same as A.S.C., just for British Cinema. Interesting thing to note is that Cinematographers working both in USA and UK can be part of both A.S.C. and B.S.C.
Cinema Audio Society.
The Cinema Audio Society was formed in 1964 for the purpose of sharing information with Sound Professionals in the Motion Picture and Television Industry. We have seven (7) categories of membership: Active (Full), Associate, Career Achievement, Honorary, Student, Retired and Corporate Members. Source
Casting Society of America. It was formed in 1982 as the American Society of Casting Directors. Best casting directors in film, television, and stage have this tag.
Directors Guild of America. It represents directors and members of the directorial team working in film, television etc.
Producers Guild of America. It represents producers of film and TV. It was originally formed as Screen Producers Guild in 1950, but later it was merged with the Television Producers Guild and American Association of Producers.
Screen Actors Guild. It was a society of main and background performers in movies & TV. Established in 1933, it was later merged into American Federation of Television and Radio Artists in 2012 and known as S.A.G. - A.F.T.R.A afterwards.
Writers Guild of America. E stands for East and W stands for West. It is a combination of two labor unions and was formed in 1912 by a collection of book, magazine, and drama writers.
As you can see, these societies refer to the specific group of people belonging to movies/TV. Like D.G.A is a society for directors and P.G.A. is a society for producers. These societies have their own motives, but they share a common goal to provide a collaborative platform for people involved in movies/TV making. Besides this, they have their own membership rules.