So you moved to LA to work in Hollywood, but when you got your first PA job you couldn't tell an AD from a gaffer? Get ahold of yourself: we talked to Jenn Semler -- a producer (er... co-producer, see below) who's worked on major productions to tell you exactly what each of those people on set do, so you're not asking the grip to do the best boy's job:
Actor (male or female): If you are lucky, the only people who show up on screen when your film is finished.
Art Director: The right, or left, hand of the production designer. The Designer decides what the world is, and the Art Director works with the entire art department to make sure that vision happens. Most of the time they also have to do all that work and keep the entire department on time and budget.
Associate Producer: To quote David Mamet from State and Main,"[The credit is] what you give your secretary instead of a raise."
Assistant: There isn't enough space on the internet to list all the things an assistant does, but here goes a try: 1. Make coffee 2. Answer phones 3. Purchase gifts 4. Book travel 5. Do script notes 6. Get their boss home safe and alive every night 7. Etc. 8. Etc. 9. Etc.