What do the little letters mean after some people’s names in the credits?

The letters after some peoples’ names generally notates membership of a professional society for their chosen craft area. More often than not, membership of these societies is by invitation only, so they are very prestigious. You can generally assume that if a person is a member of one of these societies, they have made a substantial contribution to their area of specialisation.

Casting Directors
C.S.A – Casting Society of America

Cinematographers
A.S.C – American Society of Cinematographers
A.C.S – Australian Cinematographer’s Society
B.S.C – British Society of Cinematographers
C.S.C – Canadian Society of Cinematographers
A.F.C – Association Française des Directeurs de la Photographie Cinématographique (France)

Editors
A.C.E – American Cinema Editors

Sound Technicians
C.A.S – Cinema Audio Society (Sound Mixers)
M.P.S.E – Motion Picture Sound Editors (Sound Editors)

Producers
p.g.a – Producers Guild of America