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.
Assistant Director (AD): If the director steers the ship, the AD makes sure he has the tools to do it. The AD sets the schedule and tries their VERY best to keep everyone on it.
Best Boy: Assistants to their department head (either Gaffer, see below, or Key Grip, see below below), they work hella hard and always have various types of tape and cable ties at the ready.