The Production Assistant (or PA) assists the Producer with anything they need doing. It is often a behind the scenes role.
In contrast, the Runner helps out anyone on set that needs it, not just the Producer.
You’ll see the Runner rushing off-set to the nearest supermarket to get extra catering supplies, or rushing to a nearby store to get scripts printed. One of the most important roles on set is to make sure that the cast and key crew are given food, water, and coffee.
This is a fantastic introductory position for people who just want to get their hands dirty and experience life on set.