I know it means 'per day', but is it similar to temporary or part-time employment? Or is it something like an 'on-call' deal?
Thanks!

Generally, it means that you fill in the empty slots on the organization's schedule. It's also referred to as "PRN." You have no guaranteed hours, but you also don't have to guarantee that you'll work particular hours. Win/win, in most cases.

Some places require that you work a certain number of hours in a month, but you have flexibility in which ones you take. The regular employees get scheduled, then you get a call asking which of the remaining hours you could take.

This is a great option for someone who doesn't need the benefits, such as a stay at home mom who just wants to keep her hand in the field or someone who has a full-time job elsewhere and wants to make some extra money. It usually pays quite well, since there are no medical or other benefits.